Here is what happened:
I was SO close to just abandoning the fortress, when I read that a nice dining room with statues and so on gives them happy thoughts. So I built one, all while there was practically a civil war raging. After it was done, everything calmed down, and my fortress had about 25 survivors out of ~70 dwarves that populated it once. No immigrants for two years to my "deathtrap".
Now I'm back to ~50 dwarves. All entrances are guarded by roped dogs (though no wardogs are trained at my kennel, error message "need uncaged dog" although there are plenty - what am I doing wrong?) and four marksdwarves are patrolling the parapet at any given time. But before I got that set up, about ten goblins attacked my fort - and got slaughtered by the human traders that were visiting me
Then: Big screwup!
There were two murky pools that provided me with fish, turtles and water. I wanted to bring the water a little closer to my fort, so I dug a channel, and soon after it was all dry. It hasn't rained for years, and when it did, it dried out pretty quickly. Now my map is entirely without water, so any wounded will die.
I had the idea that I could dig a funnel that leads to a small basin underground to collect rainwater, but I'm not sure if rainwater will flow at all.