Combatshield endures.
The merchants arriving amidst zombies turned out to be surprisingly capable. Not only did they kill each of the reanimated migrants, they made it too my depot. Once more, I bought all of their wood, some metal, lots of food, and a few bits of equipment in exchange for clothing and rock crafts.
Honestly, I'm not sure why the merchants keep visiting this place. They have to journey through an undead-filled desert, fight off some of the fort's reanimated inhabitants, walk past the corpses of a dozen more dwarves, and finally have most of their useful stuff exchanged for rocks and socks. I guess whatever phantom-place they spring from has an urgent need for pretty decorations.
As the merchants were leaving, six migrants arrived, boosting population to 19. The baby alpaca they brought along got sealed behind a wall, though.
A miner discovered the first cave layer! It was gruesome. At least a dozen undead (troglodytes, dralthas, cave crocodiles, cave toads...) were shambling around, and there's blood and corpses everywhere. The place does have trees, but I don't think a woodcutting expedition would be wise.
I've also completed the traditional trap-filled hallway to decrease the chances of an undead monstrosity getting inside. The design consists of two regular doors, a drawbridge, four falling stone traps and two weapon traps right now, but I'll be improving it once I have more fuel.