Just lost my second fort to the same evil Human Siege Operator Necromancer. She arrived with her army of undead (100+), surrounding the fortress walls with the stench of her walking dead.
We were off to a great start. I embarked with a band of proficient teachers and soldiers (dodging, kicking, striking, etc.) as I knew the threat of an undead siege was imminent. We embarked maybe 10 tiles away from the previous fortress that fell to this same Necromancer. The goal was to arrive prepared for battle (or at least the ability to defend the fortress). The military got a little chaotic after we kept receiving large migrant waves. At around 100 dwarves, the fortress was abuzz. Metal and sand industries churned out fine weapons, armor, and furniture. The beginnings of a cave spider silk industry was in place. The fortress was accessibly only by the main gate (with retractable draw bridge and dodge'm cliffs) and a service bridge on the main walls to allow woodworkers access to trees.
Lesson learned? Know which levers are which. I forgot which lever shut the service bridge. All of a sudden, the 100+ undead are all making a beeline toward this single door. Once they were inside, the blood of my dwarves flooded the aqueduct.
!FUN!