Actually, I don't have a hammerer. Another detail that escaped me. Thanks for the advice! And since you liked that story, there is actually a part 2, which is even more unbelievable:
While my first jail was draining out, I had my dwarves build another one across the stairway from the original. While my miners were clearing out the new cell, I saw that there was a tree growing on the z-level above, and that the miners would wind up destroying its roots. Since the tree would die anyway, I told my woodcutter to chop it down. After the old jail dried out, I had the drawbridge reinstalled and made sure it would actually close. I then left it closed, to prevent the jail from becoming an unguarded back entrance to the fortress.
Fast forward a year later, and the Plains Titan Aso Ethratunem Irum Ared (Aso Islandapes the Wave of Fog) shows up. Checking the wiki, I learn that a titan is basically an above-ground forgotten beast. This one is a gigantic flying snail with deadly spittle. Nasty.
Naturally, I raise the alert level to DORFCON 1, send the civilians to their shelter burrow and remove all the pen/pasture activity zones so that our livestock and pets go inside as well. I watch everyone file into the fortress, mentally urging them to move faster so they get inside before the bridge closes...wait, where's the bridge?! I suddenly remember that one of the dwarves that I recently threw in prison was convicted of "Vandalism," and that at the time I wondered how that was different from "Building Destruction." Now I understand: that stupid dwarf smashed the main drawbridge, leaving the main entrance wide open and unprotected.
I immediately order my stonecutters and metalsmiths to grab the nearest blocks and build a wall where the bridge was. Racing against time, they finish the wall moments after the last of the animals file in. Crisis averted.
With that problem solved, I turn my attention to killing the titan. I decide to have my battle-hardened top-notch militia squad, the Disemboweled Orbs, sally forth from the drawbridge in the old jail cell. My earlier folly has turned out to be a blessing in disguise, it seems. I check the units screen to determine where the titan is...and can't find it. "Has it left already?" I wonder. On a whim, I open the "Deceased" tab. There, sitting at the top of the (very long) list of the dead, is Aso Ethratunem Irum Ared, Plains Titan.
I am absolutely flabbergasted. How could the titan have been killed before the Disemboweled Orbs even left the fortress? More importantly, what could have killed it? Is there an even more powerful enemy that I have to worry about?
I check the latest combat report and become even more confused. It says that the titan attacked and was killed by none other than Domas Zasumstiz Aroz Anist, my militia commander and captain of the Disemboweled Orbs. This is absolutely impossible, for two reasons:
1. The Disemboweled Orbs haven't left the fort yet, and none of the entrances are open.
2. Domas Zasumstiz Aroz Anist is in prison, jailed for throwing the latest of many tantrums. (He gained his title after a previous tantrum, in which he killed the manager, the captain of the guard and a random wood burner that was in a fistfight with a child.)
I zoom to Domas and find him in his cell, alive and uninjured except for a bruise on his right thigh. Lying on top of him is the titan's corpse, and on the floor is a spatter of its ichor. I reread the combat report and discover that
1. he blocked every single one of the titan's flying spittle attacks (with his shield; apparently dwarven prisoners get to keep all of their gear); and
2. he only sustained an injury when the titan's dead body fell on him.
This is almost too incredible for me to believe. This dwarf killed the above-ground equivalent of a forgotten beast single-handedly and without letting it get in any blows, while chained to the floor.
But the kicker, and what makes this a facepalm moment, is how the titan got in the fortress. You see, Domas was in the new cell, right under the location of the tree that I had cut down when the cell was dug out. Apparently, cutting down the tree and removing its roots left a single tile of open space where the tree used to be. So there had been an extra, unguarded entrance into the fortress the whole time, without my knowledge, and my dwarves had been saved from a massacre only because their unhinged militia commander was chained to the floor right under it. No wonder Domas kept complaining about being out in the sunlight.