I know we've had a lot of threads about digging enemies, but one thing I haven't seen mentioned is requiring that the fort always have a path to it. Even outside of siege situations this could be somewhat important.
A dwarf that is trapped underground with no path to the surface is essentially buried alive. Even if he has a masterwork adamantine barrel of amontillado, he's still buried alive.
This should produce some seriously unhappy thoughts, and they should stack, such that if too many attempts to reach the surface have failed the dwarf will go insane.
Perhaps every time the dwarf goes on break he checks to see if he can escape the fort. He doesn't actually have to go outside, he just checks to see if he can reach the edge of the map. If he cannot, he gets a severe negative thought about being trapped, cut off from the outside world, or so forth. This would be enough to make a content dwarf be unhappy.
Then the next time he goes on break, he checks if he can escape. If that check also fails, he gets another equally serious unhappy thought. I'm thinking after 3 of these, the average dwarf will go insane. Maybe 4 failed checks for ecstatic dwarves.
This would mean that a siege that forces the player to bottle up their fort and seal it off from the ouside world cannot simply be ignored forever. A siege will stick around for a very long time, just camping outside. If the player attempts to wait out the siege until they get bored and leave, they could very well have a bunch of dwarves that go insane from being sealed inside the fortress.
Also the reason for checking for a path to the edge of the map every time the dwarf goes on break is both to save CPU cycles, and to give the player a little time to reopen the fort, so if you need to block off a siege until your military can grab their gear, its okay, but you can't wait for years underground.
Historically sieges often lasted for years, but the conditions for the besieged were utterly horrendous. Disease and starvation were common, eventually weakening the defenders so much that they simply surrendered. Assaults against castle or city walls were quite rare. However, as a single plump helmet farm can feel a 200 dwarf fort just fine for all eternity, because there is no disease in game, and because its trivial to feed them booze or pipe water underground, there is no reason for dwarves to come up for air.
There should be! Air shafts would be an interesting requirement, as would allowing creatures to climb down the air shafts, but thats a helluva lot more complicated than just adding in a new negative thought.
This would sort of simulate the general nastiness of being under sieges for months or years, and if the siege is not lifted somehow or other, the fortress will eventually be driven completely insane, ending in an orgy of murderous trantrums and dorfs flinging themselves into the chasm.
Also, using this you could rig up an insane asylum. Purposefully trap dwarves in rooms until they go insane, then build an arena and unleash them on each other or against caged opponents.
I'm pretty sure the most common visitor to the arenas would be nobles.