Monsterparts | Siege Rules

“Okay, the false prophet’s army arrives and begins to surround the holy city. Your walls are in decent repair but the gates haven’t been closed in a hundred years. The cathedral guard is well armed and armored but the city militia is poorly trained. Now that they’re pressing the siege, I’m going to roll…. Several thousand dice for their attacks…”

Rolling dice for every member of the attacking and defending forces doesn’t work well, but a good siege can be tactically really interesting. Let’s work out how to reduce the work to a Single Roll¹ for the attacking army and the defending city, as well as let the PCs take their own actions during the battle.

The basic idea is to reduce the number of characters to the minimum: the PCs, major enemy PCs (maybe just the commander, or for a disorganized horde not even that), and then an Attacker and a Defender with slightly tweaked rules.

The Attacker is the group pressing the siege: the false prophet’s army in the above example, or a horde of berserker Vikings, or the rampaging stormtroopers. The Attacker has an AC and HP that represents the entire group, showing the strength of the group. A well-armed army, for example, might have a high AC (for the mail armor), whereas a disorganized group of nomads might have a low AC (for their basic leather). The HP represents the size of the group: a large horde would have a very high HP, whereas a smaller army would have less. The Attacker also has attack actions that make sense: melee and ranged, probably, but maybe also special actions.

The Defender is the group resisting: the cathedral guard and the militia in the example (or be more complicated and make them separate forces, but then we’re moving out of Single Roll territory). Similarly, the Defender’s AC and HP represent the group and its size, as do the actions.

The most important point is this: the Attacker and Defender can only actually damage each other. No defending army, no matter how much it wants to, can focus all attacks on the enemy commander, and no attacking army can send every member of the horde against a PC giving a stirring speech. PCs and NPCs (such as the commander) can still attack each other (if they can reach each other), of course, but in general they will instead support the Attacker and Defender (and maybe interfere with the opposite side).

Attacker and Defender Stats

For all stats except HP, pick a sample monster stat block of an appropriate CR:

• Tier 0: CR 1
• Tier 1: CR 4
• Tier 2: CR 6
• Tier 3: CR 11
• Tier 4: CR 17

That army uses the stat, AC, and attacks of the corresponding monster.

Army HP is based on the total size: 1 HP per member per tier.

If the army has multiple branches, consider treating it as multiple armies, each with their own stats. For instance, 100 goblins and 6 ogres attack a town. The goblin army (100 hp) and the ogre army (6 hp) each make an attack on the attacker’s turn.

Support actions

• Give a rousing speech (Persuasion check, DC 15): Grants advantage on next attack
• Provide medicine (Medicine check, DC 15): Heal 1 point of damage
  • Cast healing spell: heal 1 army hit point per die of healing
• Attack: Sometimes a PC just wants to attack!
  • Does 1 army HP on a hit, or 2 army HP with a crit
  • Attack spells do 1 army HP per spell level
    • Group spells like Sleep count as attacks for this
• Cast a Spell: depends on the spell. Attack spells as above, buff or other spells, use a judgement call.

Preparation Actions

Attackers and Defenders might know there’s a siege coming (the attackers has better know), and so can take actions beforehand to prepare.

• Reinforce walls (Engineering/Int check, DC 15): Increase army AC
• Ambush: Stealth vs enemy’s passive insight, grants surprise
• Rig up a trap - Survival check vs enemy passive perception.
  • Deals hit points of damage based on a single die roll. Die size decided by the scale of the trap, with d4 for a small trap (small pit), d12 for a big trap (collapsing wall)

Morale checks

When each side’s HP drops past certain points (50%, 75%, 90%), or when a named character (PC or NPC) is killed, that side makes a Morale check (Leader Cha, DC 15/12/10). If they fail the check, then that side breaks and runs: the Attacker breaks off the siege and flees, or the Defender abandons the walls and tries to hide in the city or run away as a disorganized group.