Just an update here...
Finally managed to chase the goblins that camped outside my entrance off (at least made them move off a bit), and started collecting bodies that had been caught in the crossfire etc... Nice to have some breathing room!
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"A Vile force of Darkness has arrived!"
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You got to be kidding me! I can't handle one siege, and now I got two at the same time?!
So, without further ado, I decided to give up. Go out in a blaze of glory. I'd had a good run with this fortress, couldn't last forever I guess.
I assembled my troops, lowered the bridge and met them head on (we're talking around 70 goblins and trolls now, versus my remaining ~12 dwarves with various experience).
Didn't think they'd last long. Surprisingly enough though, they went out and kicked ass!
Or well, a couple of them did. Some just died right away. Others went martial trance-crazy (had one guy who had 3 in a row, his kill-count from that fight alone is around 25 kills).
A squad of trolls got past the defenders though and entered the fortress, and here I thought I'd see lots of dead civilians...
Surprise again! Apparently my civilian dwarves got tired of doing nothing, so they decided to swarm the trolls and beating them to death with their fists. Lovely show! A few of them died though, but it's all worth it!
They didn't get all the invaders, but boy, a goblin on the run sure is fast!
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Thanks for all the advice everyone, I'll make sure to try my best to be better prepared the next time, and judging by the display I just witnessed, I think I've underestimated my dwarves quite a bit!
[Edit]
Huh, seems I was a bit rash in declaring myself having survived... Still lost about 20 dwarves to the entire ordeal, and while that might not be too bad considering the circumstances, these dwarves were, apparently, related to a lot of dwarves in the fortress, so now I got a massive tantrum spiral. !Fun! !