Still playing .18 with rephikul's IM .12 (I'm so behind the times) and FD. My first successful fortress.
Well, I had about three years of continuous sieges, which meant no trading (whatever, didn't really need it), and no migration. Between the years 103 and 106 (3rd to 6th years of my fort), the population dropped from some 120 to 50. A wave of migrants brought it back up to 60ish, but then a party of Nightwings got into my fort via the open-air decomposal zone while my tiny, inexperienced military (actually, the original militia commander was still around--Legendary Mace, Fighter, Armour User, and really good at everything else) was out front dealing with some stray Putrid Blendecs that got past my defences. By the time I finally got the 5-dwarf militia back into the danger zone in the fort a dozen dwarves had died.
Multiple sieges are fine, but the pathing for Furies isn't very good, so they usually just end up moving for about 30 steps then stopping for a few seasons. After the Nightwings were dealt with, I sent out my militia to clear out a couple dozen of the Furies, as well as a half-dozen pathing-stuck Nightwings. Somehow, the fighting went all the way over to a murky pool, and four of the dwarves drowned in it... (The last became a pincushion despite candy equipment.) I rage-quit and decided to build a new, better fort, with what I've learned.
This new fort will have magma for those invaders who fall off the bridge entrance, an open-air decomposal area combined with statue garden that will have walls high enough for the flyers to be discouraged (I hope), and a better military. No more dralthas killing a dozen dwarves.