August 16, 2017

Advanced Fortifications

Variant Rules
Notes from the Nails: this started out as a sidebar for the subclass we're publishing on Friday, but it ended up being its own thing!

Advanced Fortifications

Castles and other defensive structures are integral to the high fantasy experience, so it seems strange that only passing mention is made of them in the DMG. This article is an attempt to expand those rules and add some interesting variations.

The Anatomy of a Castle

Let's start with the fortifications themselves. There is a potential for massive variation between castles, depending on where they are located, what their purpose is and who built them. In particular, size, shape and material have a large impact on how much a castle costs to build and how difficult it might be to assault.

Types of Castle
This list includes information from chapter 6 of the DMG.

Castle Construction Cost Construction Time Maintenance Costs Garrison
Blockhouse15,000 gp100 days50 gp/day40
Fortress 1,000,000 gp1,800 days1,000 gp/day400
Hill fort5,000 gp60 days50 gp/day25
Large castle500,000 gp1,200 days200 gp/day100
Medium castle300,000 gp800 days 150 gp/day75
Ruined castleN/AN/A100 gp/day 50
Small castle50,000 gp400 days100 gp/day50
Watchtower15,000 gp100 days25 gp/day10
Wizard's tower25,000 gp125 days50 gp/day5

Blockhouse. A small building that provides cover for ranged fighters but no accommodation for civilians. Designed to be constructed quickly and cheaply; unlikely to last long against a determined assault. Could be considered as a low-rise watchtower.

Fortress. A large, fully-militarised structure with strong defenses and significant firepower. Scientific principles are applied in their design.

Hill fort. An early type of castle where the primary defense is a wooden palisade around a hilltop, surrounded by earthworks.

Wizard's tower. A wizard's tower is usually taller and less heavily built than a watchtower, but may include unusual arcane defenses. The garrison are likely to be apprentice wizards or summoned monsters.

Additional Security
Beyond the basic structure of the castle, various additions or changes can be made to increase its strength. The following can be applied to any type of castle at a cost equal to the stated percentage of the base castle's construction cost.

Feature Cost 
Deathtrap 100 gp per individual trap; 1,000 gp for a complex trap-chamber. 
Dungeon25% of base construction cost.
Escape tunnel10% of base construction cost.
Gargoyle defenders500 gp per gargoyle.
Increased garrison2 gp/day per soldier. 
Magical traps200 gp per trap
Moat (dry)25% of base construction cost.
Moat (flooded)35% of base construction cost.
Mountaintop location Base construction cost and time are doubled.
Round turrets10% of base construction cost.

Deathtrap. Some castles are built specifically to kill intruders in fiendish complexes of dead ends, trapped corridors and drowning chambers. Individual traps might include the hidden pit, collapsing roof or fire-breathing statue from the DMG, while a self-sealing room that slowly fills with poison gas would count as a complex trap.

Dungeon. A resilient, underground complex that provides shelter in times of hardship. Dungeons cannot be damaged by surface-based attack and fire cannot spread from above ground into them.

Escape tunnel. When the castle is besieged, an escape tunnel enables the defenders to retreat in safety. The length of the tunnel could be anything from a few hundred feet to several miles.

Gargoyle defenders. Some of the castle's decorations are replaced with dangerous creatures.

Magical traps. Arcane glyphs have been inscribed at key points in the castle, presenting an additional hazard to besiegers. Magical traps use the rules for the glyph of warding spell. Spell glyphs could contain any spell of third level or lower.
Moats. A ditch surrounds the castle, making it harder to get close to the walls. A dry ditch is filled with stakes that make it impassable for mounted troops, and the steep slope requires a DC 15 Strength (Athletics) check to climb. Flooded moats can be swum across, but swimmers are vulnerable to attack and unable to carry much equipment with them. Aquatic monsters such as quippers, water weirds or hydras could be brought in to live in a flooded moat, but the cost to do so is usually prohibitive.

Mountaintop location. Building a castle on the top of a mountain is a logistical challenge, but is worthwhile because the natural defenses and steep escarpments make assaulting it a daunting prospect.

Round turrets. Square turrets are vulnerable to attack by siege weapons, as the corners can be smashed to compromise the structure's integrity. Rounded towers are harder to break down, having no obvious weaknesses. They have resistance to damage from siege equipment.

Siege Rules

By and large, siege warfare can be carried out using the standard rules. Armour Classes and Hit Points for objects are detailed in the core books, as are the rules governing climbing, falling, swimming and traps. However, some of the situations involved in attacking or defending fortifications are not covered or are left ambiguous. Here are some rulings I would make:

AC & HP for Structures
Permanent structures, such as walls, towers and buildings, should be broken down into 10 foot cubes for the purposes of HP and damage: attacks against part of a wall destroy only that part (though unsupported sections of wall above a destroyed section may well collapse shortly afterwards). Based on the information in the core books, I would use the following statistics:
Hit Points. Walls have 30 HP per inch of thickness. The walls of a standard building might be 6" thick. Fortified buildings might have walls 1 or 2 feet thick, while the curtain walls of a castle are likely to be at least 5 feet thick if made from stone (that would be 1,800 HP per section). That may seem a lot, but there wouldn't be much point to building walls in the first place if they couldn't resist attack!

Armor Class. The ordinary armor classes for materials listed on page 246 of the DMG can be used for most structures, but some parts of a castle are likely to be reinforced using special materials or techniques. Wooden doors banded with iron should have an AC of 16 or 17, for example. Earthworks might have an AC of 12 (though they can be made very thick to compensate). Lastly, in areas where stone is scarce, castles may be built from ceramic bricks. I would give this an AC of 15.
Burning Buildings
Although the strongest fortifications are made of stone, wooden buildings are also used where resources are scare, time is short, or no attack is expected. If such a building is set on fire by any means, the fire can spread quickly. I would rule that an unchecked fire would fill the room it's in after 1 minute. Any creature in the room would then take 2d6 fire damage at the start of their turn. The risk of the fire spreading to a neighboring room/building could be represented by a d20 roll, with modifiers to account for wind direction, fireproofing, etc., as appropriate. A wooden building that has been burning for 10 minutes starts to collapse.

Collapsing Walls
Fighting in and around castle buildings presents the possibility of large quantities of masonry collapsing onto people, or people standing on top of a wall being injured as it crumbles. As a general rule, when a creature is standing on a section of wall that is demolished, I would treat that as if it was falling, but double the amount of fall damage. Creatures that are standing within a certain distance of the foot of the structure - probably half its height to either side - would be subject to a flat 4d6 bludgeoning damage (DC 13 Dex save for half).

Killing Fields
In most castles, towers and baileys are set in such a way that attackers are exposed to constant attack from defending archers. Figuring out how many archers are able to fire and rolling their attacks individually is impractical, so instead I would have all attacking creatures that start their turn in a zone overlooked by garrisoned towers roll a DC 13 Dexterity saving throw. They would then take 2d6 piercing damage on a failure or 1d6 on a success.

Spiraling Staircases
Spiral staircases can be either right-handed (turning clockwise from the ascender's point of view) or left-handed (turning anticlockwise) - most are right-handed. I would say that melee attacks made by a creature whose handedness matches the stairs against a target on a higher step have disadvantage. If a character's handedness has not been determined previously, roll a d100. On a roll of 1-88, the character is right-handed, on 89-99 they are left-handed and on a 00 they are ambidextrous.

New Siege Equipment

Although the DMG covers this one pretty well, here are a few additional toys for you to play with.

Arcane Cannon
Large Object

Armor Class: 17
Hit Points: 50
Damage Immunities: poison, psychic

Arcane cannons come in many shapes and forms, depending on the whims of their creators. They are deployed much like gunpowder cannons, but fire a blast of magical energy instead of a conventional projectile.
     Arcane cannons are usually mounted on a wheeled carriage. Before it can be fired, an arcane cannon must be charged and aimed. It takes one action to charge the weapon, which must be performed by a spellcaster, one action to aim it and one action to fire it.
     Arcane Beam. The cannon projects a beam of energy in a line 120 feet long and 5 feet wide. Any creature in the area must make a DC 15 Dexterity saving throw, taking 21 (6d6) force damage on a failed save, or half as much on a successful one.

Flame Dragon
Large Object

Armor Class: 18
Hit Points: 100
Damage Immunities: poison, psychic

Flame dragons are hollow stone statues mounted on wheels, containing mechanisms and fuel that allow them to belch flames at nearby targets. Mainly used against invaders after they have breached the internal spaces of a fortress, a well-placed battery of flame dragons can turn a courtyard or hallway into a impassable hellscape.
     A flame dragon holds enough fuel for five flame blasts, but requires three actions to refill. It must contain fuel and be aimed before it can be fired; aiming and firing require one action each.
     Flame Blast. The dragon sprays flames in 30-foot cone. Any creature in the area must make a DC 13 Dexterity saving throw, taking 13 (3d8) fire damage on a failed save, or half as much on a successful one.
     Explosive. If a loaded flame dragon is destroyed by fire damage, it explodes. Any creature within 20 feet of the weapon must make a DC 16 Dexterity saving throw, taking 28 (8d6) fire damage on a failed save, or half as much on a successful one.

Rocket Battery
Large Object

Armor Class: 14
Hit Points: 50
Damage Immunities: poison, psychic

A rocket battery takes the form of a wooden or metal rack, from which dozens of rockets can be simultaneously launched. While they can be difficult to aim, the characteristic screeching sound of the rockets can be extremely intimidating, causing all but the most disciplined troops to dive for cover.
     Rocket batteries are usually light enough to be carried by a single humanoid, and may or may not be wheeled. Non-wheeled versions require one minute to limber or unlimber. Before it can be fired, the battery must be loaded and aimed. It takes two actions to load the racks, two actions to aim and one action to fire.
     Rocket Barrage. The rocket battery aims at a point within 600 feet. Any creature within 40 feet of that point must make a DC 15 Dexterity saving throw, taking 7 (2d6) fire damage on a failed save, or half as much on a successful one. Furthermore, any creature within 60 feet of the target must make a DC 15 Wisdom saving throw or fall prone and be frightened until the end of their next turn.
     Explosive. If a loaded rocket battery is destroyed by fire damage, it explodes. Any creature within 20 feet of the weapon must make a DC 16 Dexterity saving throw, taking 28 (8d6) fire damage on a failed save, or half as much on a successful one.

Special Ammunition

The special types of shot described below can typically be used by any ranged siege weapon. Liquid munitions are for use with cauldrons.

Alchemist's Fire. 16,000 gp per cauldron-full.
Although incredibly expensive, a cauldron full of alchemist's fire is likely to spell doom for whatever it is poured on, no matter how tough they may be. This functions exactly like boiling oil (see DMG 255), except that it does 12d12 fire damage instead of 3d6.

Boiling Wax. 30gp per cauldron-full.
Boiling wax works much like boiling oil, except that it deals only 1d6 fire damage. Targets that fail their Dexterity save are restrained for one minute, though they can attempt the saving throw again at the end of each of their turns.

Breaching Shot. 25 gp per round.
Designed to break down fortifications, a breaching shot might include a charge of corrosive acid, carry a minor thunder enchantment or be cunningly shaped to penetrate walls. Attacks with this munition deal double damage to objects and structures; this can be stacked with similar abilities possessed by the weapon or its crew.

Guiding Shot. 100 gp per round.
This piece of ammunition is imbued with divination magic that enables it to be used as a beacon to guide other shots towards the same target. If you hit a target with this shot, any attacks by the same siege engine against the same target within the next minute have advantage.

Holy Water. 100gp per cauldron-full.
Holy water is harmless to most targets, but against fiends and undead, it deals 4d6 radiant damage. Any fires or burning creatures in the area are immediately extinguished.

Incendiary Shot. 50 gp per round.
Most siege engines can fire burning shot -- ballistae bolts with oil-soaked tows, barrels of pitch for a trebuchet or bucketfuls of burning coal in a catapult. These munitions all deal their conventional damage as usual, but then burst or explode, scattering flames in a small area even if the original attack missed. Any creature within 10 feet of the target (or within 10 feet of the edge of the weapon's normal area of effect) takes 3d6 fire damage, or half as much on a successful DC 13 Dexterity saving throw. Flammable objects in this area catch fire.

Quicksilver. 1,000gp per cauldron-full.
When quicksilver is poured onto creatures from a suspended cauldron, it deals no damage, but they must make a DC 15 Constitution saving throw or be poisoned for 24 hours. Creatures that fail their saving throw must also roll once on the long-term madness table. The 10-foot square area that the quicksilver was poured on becomes difficult terrain for the next minute.

Scatter Shot. 10 gp per round.
Any siege weapon that makes an attack roll against a single target can be modified to target an area using some form of shrapnel or grapeshot. Instead of making an attack, choose a point within range. Any creature within 10 feet of this point must make a Dexterity saving throw or take half of the weapon's normal damage (they take a quarter of the normal damage on a successful save). The DC for this saving throw is equal to 8 + the weapon's normal attack bonus.

16/08/17: extra ammunition types added, HP & AC for walls clarified.


  1. Great work as usual, and definitely useful. I recently had to check wall hp and similar details in the DMG and was angry to discover it includes almost nothing related to large scale warfare. This article does a great job at fixing it.

    I do have several questions/suggestions:

    Why are the DCs 13? just as a compromise between 10 and 15 or is there something else?

    I think charging the arcane cannon should probably require using a spell slot, unless the cannon itself holds the magical energy and just needs to be activated.

    As for the staircases: disadvantage seems a bit harsh. A spiraling staircase where the enemy's weapon is slightly blocked seems more like a half cover.

    1. DC 13 seemed to come up a lot in the DMG for this sort of thing. I don't know. 15 seems kind of harsh? But DMs should feel free to modify DCs based on their own needs.

      I was imagining that arcane cannons would hold their own charge - or that they could be loaded with energy cartridges or something. I feel like they wouldn't be worth installing if you had to spend a spell slot for every shot.

      Disadvantage is so easy to use though! Half cover is fine, too, if you prefer.

      Is there anything else you feel is missing from the DMG? I'm sure there are thing's I've missed here...

    2. Mostly, I'm missing hp and ac for walls, towers, etc.

    3. Hmm. On the one hand, that uses the rules in the DMG.

      On the other hand, I have a hard time believing the tarrasque would require 6 rounds (it deals ~150 damage per round, and double to objects) to destroy a 10-foot section of wall. It would take 7 meteor swarm spells to destroy a 1800 hp wall. These amounts of hp seem too much, rendering walls almost indestructible.

      So I re-read the DMG section on object hit points, and it gives ~30 hp to a large object, no matter it's thickness. Based on this, I'd give a wall around 30-50 hp per foot of thickness, which reduces it's hp to 100-250. That seems much more reasonable, both for siege weapons, spells and monsters (while remembering the damage threshold prevents most other damage).

    4. I hear what you're saying, but stone walls *should* be strong. I've visited my share of castles, and they are BUILT, if you know what I mean.

      And thinking about it, 6 rounds is 36 seconds. Ripping through a wall in less than a minute is exactly what I'd want the Tarrasque to do. "Quickly, we have to stop it before it breaches the walls!" would be a great encounter. Meteor Swarm *should* be wasted against the strongest parts of fixed fortifications - or what's the point in building them?

      What needs to be borne in mind is that you don't attack castles by battering the walls down, you go for the weak points. Gatehouses, secret tunnels, posterns, sewage pipes, treacherous agents on the inside, the roof... literally anything other than the walls.

    5. Hmm. I guess that's a fair point. Also, the walls where there are turrets, ballistas, archers etc would be thinner to allow them to shoot, and goristros, earth elementals etc should really be pointed towards gates (most enemies would prefer to leave the castle itself mostly intact anyways).

  2. Nice. I wasn't planning on using sieges in my campaign, but now I might.
    In the ammunition section, you mention liquids, but don't actually provide any. Is this a mistake, or intentional?

    1. Good point! I was going to write some special liquids for cauldrons, but then totally forgot to. Any suggestions?

    2. Elemental stuff like ice

      Acid and Alchemist Fire. like the PHB, but more

      Holy Water to deal with hordes of zombies

      A toxic gas heavy enough to be stored in a pot.

      Quicksilver (Mercury). Poisonous and looks awesome, if expensive

      A cauldron of endless water, that creates enough of a flood to push enemies back

      A cauldron of healing potion to dump on your allies.

    3. Liquid gold that burns, then hardens (like The Hobbit)

      Druidic Moonwell water. Harms most, but heals those pure of heart

      Grease, like the spell, to leave your enemies unable to stand.

      I can do more if you'd like

    4. Burning oil (for fire), boiling oil (make walls slippery to prevent it's boiling), tar/morter/concrete (something thick and viscous and stops/slows movement), water (to drench people and soak munitions), acid (cuz why not?), molten metal (cuz wo wouldn't love a corpse iron sculpture?)... uh, that's all I got.

    5. Well, I've added a few of those now. Great ideas, everyone!

      For acid, I'd treat that the same as oil, just a different damage type. For oozes (I'd restrict it to Large or smaller), it's not really an attack, you're just adding a defender. Maybe make them immune to falling damage when deployed from a cauldron? Molten metal seems like it'd be difficult to keep the cauldron hot enough to be useful, but if you have magic or something, you could base it on the boiling wax stats above. Grease I've folded into quicksilver. A Cauldron of Endless Water would be a unique magic item, not something you could do with any old siege engine; I was planning to save things like that for later.

  3. Hail, just a question...
    Round Turrets, should be Round Towers?

    1. Either, really. When I say 'round turrets' I'm imagining something like Bodiam Castle:

  4. mmm... Good brain food. This would be a great PDF and I could see myself using this as a... 5E aid to go with my Stronghold Builders Guide from 3.5. Not everything converts perfectly and this is a good one or two page guide for all you NEED for building, but there are extra things that, well, the 130 page SBG just has MORE. This is perfect for on the fly and initial building for a party and gives a DM some staying power for a surprise by the players until they can do the proper research.

    LOVE it!

    1. Yeah, that's basically what I was going for. Glad you like it!

      I'd have liked to see this as a pdf too, but the Finger has a lot on his plate and I don't really have the graphic design skills to pull it off. Maybe I'll try to teach myself one of these days...

    2. consider using the homebrewery. it makes really nice PDFs

    3. What about resistances and immunities? The DMG speaks a little, mostly just poison and psychic does nothing, figure out the rest on your own, but there are some weird situations to me. Like radiant and necrotic damage, I feel that should EVENTUALLY work on a castle wall, but is resistance still not enough? I mean the sun on a brick will break it down eventually, but not for millenniums.

    4. Intersting question (apologies for the late reply). My first instinct would be not to overcomplicate things. Like, for radiant damage, the things that deal meaningful amounts of it are more than just a mild sunburn. It's more like pillars of searing flame, streaking down from the heavens; angels wielding divine swords that cleanse the impure. Necrotic damage is something that constructs might be likely to resist, but even so, they'll probably have some organic elements that can be compromised. Rubber can perish; wood can rot; iron can rust.