Obviously for small lairs, the cost of a siege operation would quickly outpace the benefit. But for large lairs of the mega-dungeon variety, the treasure you’d get after a successful siege should easily cover the cost of the mercenaries and fortifications you needed, right?
It seems to me that you have a few dungeon/lair categories:
A.) Raidable with current party & henchmen, not worth raiding.
B.) Raidable with current party & henchmen, worth raiding.
C.) Unraidable with current party & henchmen (TPK likely), not worth raiding.
D.) Unraidable with current party & henchmen (TPK likely), worth raiding.
Reasonably, A would be passed by, B dived into, C passed by, and D would be a case where a siege might be considered.
Has anyone played a siege at various character levels? I think a siege would take a sizable mercenary company at least, so 1st level chumps probably won’t be organizing any, although they might convince a local authority to do so.
I have the feeling that dungeon/fortress sieges would be a common activity and a significant source of income for domain holders, as smaller lairs and dungeons wouldn’t pose a significant threat, but megadungeons/fortresses could project power and threaten the domain holder’s rule.
Is there a good way for determining more precisely when a siege might be profitable?
Not necessarily a whole fort. Could be just a palisade wall or something, I think.
Basically I’m considering it as a means of killing off everything in the dungeon so that the PC/Henchmen have no reason to retreat from anything that survives once they finally go inside to clean up.
It would also depend on the number of entrances, and how far spaced apart they are. Most megadungeons have at least two entrances, and they’re sometimes in different hexes.
As others have mentioned, ecosystem and multiple entrances are likely going to make the largest dungeons difficult to siege since it would dramatically increase the time it takes to starve out the denizens (assuming it was possible at all). Every month one has to keep a healthy mercenary regiment up is going to eat into the profits. That’s not necessarily a bad thing, it just shifts the risk/reward slider towards the safer result, where the PCs are essentially re-investing their money into ensuring their survival. In essence, this is comparable in ACKs to spending money on your XP reserve.
The DM could likewise take the story in any number of directions as a result: the desperate monsters dig too deeply (perhaps too greedily) to find food and escape, only to unleash something much more evil that can put up a decent fight to the entrenched force.
So, looks like there are a few ways an attempted dungeon siege could go pear-shaped:
The dungeon inhabitants ignore you and eat fungus until you run out of cash.
You end up playing whack-a-mole with new dungeon entrances/exits dug by mining monsters.
You miss some entrances entirely and the sneaky monsters avoid your blockade.
Maybe there’s a sweet spot in there where the dungeon is just big enough to have treasure that would cover a modest siege, but not so big that you’re worried about missing entrances or newly-dug passages… also, there are clearly setting-specific details concerning subterranean ecosystems that would be relevant to the siege calculus.
If there are edible fungus that grow in darkness and that dungeon dwellers can survive on, why doesn’t every castle-builder include a whole farm of the things in the cellar or keep in case of a siege…?
If the fungus is nourished by the Chaotic Energies of the Deep Dark below All That Is, surface castles may face two difficulties, (a) prohibitive costs to dig deep enough to grow fungus in any respectable amount, and (b) the things you find while digging that deep.
Or perhaps the fungus mutates the eater in small, but noticeable, ways. Goblins don’t care, but humans certainly will.
Or perhaps continued consumption of the fungus alters alignment toward Chaos.
A dungeon built by a mage within their domain is effectively under siege if the mage pays the extra garrison cost to compensate for this source of monsters near the peasants.
When I run adventures that have a keep and a wilderness map showing that it isn't surrounded by farmland (Keep on the Borderlands and Keep on the Shadowfell come to mind) I often do have the inhabitants herding cattle in the courtyard and using the dung to farm mushrooms in the basement. This does typically imply a shift toward Chaos as I describe how horrible it is to eat the rations from a place where everything is made of fungus, and town encounters often reflect the tensions between the healthy, tanned ranchers and the pale, resentful gong-farmers.
