Well, portalrasped has fallen. Suprisingly it had almost nothing to do with enemy action. It was down to my own dorfiness really... I hit upon a smart idea of crating my own chokepoints using the first soil layer which i had only really utilised for a few small treefarms. I drafted a load of dwarves into a masonry corp, and proceded to build the first one, a small distance from my main way in. It consisted of a bunker with twin ballistae firing down a 40 tile walled walkroute, with ramps taking the walkway under this bunker through a heavily trapped killroom. this killroom had firing points for marksdwarves around it. above the bunker was a firing platform for more marksdwarves. construction was completed very quickly, and i felt i needed to adda ton of new masonry to merge my old firing tower entrance to the new beast of a gatehouse. this required the removing of exisitng masonry. of course, when ripping out some now obsolete fortifications i totally forgot that they were supporting a pretty large room for food, booze, ammo and the roof above it. this promptly suffered from too much gravity, and a massive cave in ensured. this killed: 18 marksdwaves stationed on the old tower while the new one was finished, 10 children tht were gleefully ripping out the old fortifications, 5 masons working on the bunker below, 4 engineers trapping the cavern beneath the bunker, a whole load (more than i could be bothered to count) war animals and animals waiting in the kennels (z level below...) and the entire population of my hospital, which at the time as 4 medical dwarves, 2 random sleepers, and 4 wounded troops (well, 2 of them were cripples with no legs). there were many more serious injuries, but with a hospital full of caved in bunker i could do little to help them.
needless to say the survivors (this accident reduced my fort to 60ish...) were not happy with the loss of so many freinds and children. mass tantruming ensued, dorf on dorf violence broke out, and with a much reduced military the arrival of an FB then a siege (which waltzed and flew through my collapsed defences) was too much to take. the last to die was my baroness, who sensibly decided the best place to hide was in her golden mausoleum.
so, to conclude. dropping a huge firing tower through your hospital is funny, but has negative consequences. lesson learnt - check the z-level above before hitting d-n-enter...