Argh. My current fort is on a grand megaengineering mission; to construct a series of locks that can lower a ship (imaginary, of course) from the ocean down into the various cavern levels below. I penetrated the aquifer and dug out a giant vertical shaft to the top of the upper cavern, which turned out to be a yawning chasm. I started a nice little fortress down there carved out entirely within a natural pillar standing 34 Z levels tall in the middle of the biggest open space I've ever seen underground, and began planning out how ships would get to the cavern floor from up there. I'm thinking I might construct a canal bridge across the middle of the open space. It'll be fantastic.
Then the goblins came. Just a couple of ambush parties, I figured it wouldn't be too hard. I had thirty dwarves at arms with decent enough equipment (though somewhat haphazard - salvaged and purchased, not made from scratch) and five legendary warriors among them. The ambushers descended to the bottom of the future lock shaft and I launched an ambush of my own, sending my troops in to hit them while they were still down in the canal.
I have no idea what went wrong. It should have been a slaughter, but instead it was a slaughter - my entire military was wiped out the the loss of only a handful of goblins. I was so shocked that I almost didn't think to lock the granite doors before the goblins got out of the access passage and into the caverns proper. But I did, and I was left at an impasse. The entire military might of my fort was dead and all my weapons and armor were lying in the charnel pit that the future canal bed had become. I suffered a minor tantrum spiral among the survivors, nothing fortress-ending but enough to keep my masons busy making coffins for a while. This being an ocean-shore map, I didn't think I had a good supply of water either - I had to quickly build a construction-lined reservoir and a pump to fill it with. I later saw a dwarf take a drink directly from the water at the underground shore, though, so perhaps I didn't need to panic about that - maybe cavern water isn't salty since it's sort of its own biome down there. Anyway. To make a long story short, I stayed sealed up underground for over a year licking my wounds and preparing a massive hall of traps. When I was finally ready I opened the doors, and the patient ambushers came pouring in to jump into my cages. Hooray! And just in the nick of time - a migrant wave arrived literally as the last team of ambushers decided to flee rather than fight.
I started a massive cleanup operation. The goblins had killed a band of human traders, and there were the corpses of my own military to deal with. Lots of work. Finally I was starting to get back to the task of canal engineering again... and then a Forgotten Beast showed up that was made of vomit, and thus was unkillable due to the current version's bugs. It came out of the worst possible cave access, too, there wasn't any way to wall it off before it reached my fort.
Argh. I'll see if my previous seasonal save is worth going back to, otherwise I'm going to shelve this save until .04 comes and see if that makes it possible to kill that thing.