OK, my last siege didn't go so well. I won, but my military was cut in half (though they took out about three goblins/trolls for every dwarf that went down).
So, since I didn't have time to get a new military up in time for the next siege, I planned ahead and made sure I could wait out the next siege. I built a couple of bridges across the river that cut my map in half (getting rid of the old floor across it that I was using), hooked 'em up to some levers, and made sure everything I needed was inside (sadly, I didn't have time to enclose my pastures either).
Anyways, the elves show up to trade, and shortly after they get inside "A Vile Force of Darkness Has Arrived!" After the siegers all get on the map, they're over 100 strong (including mounts), which I could never have handled. After wiping out all of my pastured animals, all of the siegers are on the opposite side of the river from me, except for the general's mounted squad. So, seeing a good opportunity, I pull up the bridges, leaving only 16 mounted combatants on my side of the river.
The goblin general, mounted on his giant toad sits with his squad near the edge of the river for a while. I come back a while later and the general is on the other side of the river somehow. "Huh? ... Oh! He's mounted on a giant toad! That's probably how. Hmmm... wouldn't it be funny if..." The words were barely out of my mouth when the toad crossed halfway back across the river with the general astride it, and it stopped there. In the middle of the river. Underwater.
Yeah, the general of the army attacking me drowned in the river because of his stupid amphibious mount. I've never facepalmed in joy before.