nothing. its a common bug, the siege tag generally ends after a season or 2, no more than a year generally. happens all the time with zombies and other sieges. I try to not close off my base completely until they are very near, they don't move too fast anyway, and will wander in a direction to reach a dining room/meeting area. if their is not a meeting area or dining room that can be pathed too, they hide and then never exit hiding (undead especially). I think this is both a raw error and a vanilla DF issue. I'm not very far into understanding creatures, but it seems that something about undead lets them hide, but something else about undead refuses to let them ambush... so don't close up base without them close enough that they can't become hidden. try to keep a tower central above any gate/entrance system with training warriors(I prefer melee warriors, archers piss me off). if you lock the guards into the tower (hatch/door forbidden) and they can't climb over the wall (a roof over the room) and they can't get hit through a fortification (fortification outside, window inside that way they can see/ but not shoot or be shot) because if they do get shot at they can(which by murphy's law means they will) jump into the fortification to dodge then will go outside the fortification to reach the enemy. The guards will be on alert that there are enemies right outside and the enemies wont be able to hide + the enemies will just meander right outside the gate waiting to get access to the guards they can see.
Similarly build an artifact item in range of the tower and watch as every titan/megabeast/dragon (+many siege units) sits there trying to destroy it while you build up your army inside.
I've actually built walls and a roof over one once, just had to maintain about 11 squares distance to keep from having whoever you send to build from canceling the job or charging the target. afterwards used it as an archer training facility (it was a bronze titan).
its a pain. Everyone has had it happen at least once.
