Dark fortresses, those are the gobbo prison sites, as I call them? The "box office"? With angels/"stars" later on? I'm coming up with new spoiler terms, I guess...
EDIT: The things which have builtin security unless you `exterminate TROLL` because of how much lag they generate?
They are the Slade spire version of the Goblin children-correctional-institutions or Troll-infested-lag-fests.
The Goblins aren't even the main problem as mentioned before, the Trolls below are what causes all the lag. With my custom race - it uses Dwarf sites as a substitute - I had over 4'600 Trolls in 3 chunks (?!) sitting in the cave layer(s) around the fortress...("exterminate TROLL" was the only way to get out of there it felt like...)
What annoys me is that they don't even attack you on sight. It's like, you have to kill each and everyone of them by using the aimed attack...and even when you slaughtered a few dozen of them they still seem unfazed by your genocidal intentions towards their population. One time, playing a Necromancer Dwarf, I just raised the skeletons around in their cave and watched them be slaughtered by them. Still didn't reduce the lag, considering that out of the several thousand I maybe killed 500 in the end.
About the empty dark fortresses, ... I had some of those as well, seems like sometimes they just get abbandoned. Doesn't even matter if it is a "slade spire"-one (-> stairs) or a normal stone one, but that latter seems to have that happen to it more often.
I really wish you could travel closer to the Goblin sites and there weren't a few thousand Trolls in them...overall they are unnecessarly overpopulated for no reason, making them lag horrendously, it's not even like the Goblins pose much of a threat: after killing a few of the Goblins, the rest runs away in terror. And the Trolls, as mentioned before, don't even mind it when you killed their family, friends, distant relatives, colleagues and pets.
Still weird that the structures change their room layout upon entering and leaving, have yet to test that more thorughly though. Haven't been to the lag spires lately and my newest adventurer (custom race) is just about equipped and ready for venturing out.