TEXT: Advanced Fortifications
  C
  astles 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
  Blockhouse
  15,000 gp
  100 days
  50 gp/day 40
  Fortress
  1,000,000
  gp
  1,800 days
  1,000
  gp/day
  400
  Hill fort
  5,000 gp
  60 days
  50 gp/day 25
  Large castle 500,000 gp 1,200 days 200 gp/day 100
  Medium
  castle
  300,000 gp
  800 days
  150 gp/day 75
  Ruined
  castle
  N/A
  N/A
  100 gp/day 50
  Small castle
  50,000 gp
  400 days
  100 gp/day 50
  Watchtower 15,000 gp
  100 days
  25 gp/day 10
  Wizard's
  tower
  25,000 gp
  125 days
  50 gp/day 5
  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.
  Dungeon
  25% of base construction cost.
  Escape tunnel 10% of base construction cost.
  Gargoyle
  defenders
  500 gp per gargoyle.
  Increased
  garrison
  2 gp/day per soldier.
  Magical traps 200 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 turrets 10% 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
  B
  y and large, siege warfare can be carried out
  using the standard rules. Armour Classes and
  Hit Points for objects, including wooden and
  stone walls, 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.
  Under Siege by guang2222
  
  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.
  Helstorm Rocket Battery by Unknown
  
  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.
  Crusader by Unknown
  
SOURCE: textdata/thevault/Dungeons & Dragons [multi]/5th Edition (5e)/3rd Party/Mage Hand Press/Non-Published/Advanced Fortifications.pdf