I recently got one.
I settled in a "warm climate" area, savannah and a coast, but was surprised that pools and ocean freeze during winter. Two months or less, but still. I even get snowstorms.
When the zombies came in winter, I locked down the site and prepared a mix of traps and a bolt door. The ocean froze in the meantime, and zombies started walking over it. The ice thawed and one zombie plunged to the bottom (together with tons of ammo the zombies were shooting everywhere). By summer I prepared the traps, opened the bolt door and cleared the zombies. Except that one at the bottom who wasn't attracted to my bait. Face palm moment was when I realised that out of the three ocean levels, only two lowest had the ramps. The zombie couldn't get out! Apparently, it is also impossible for it to climb underwater, but I wasn't considering it, I just "saw" the ramps which weren't there.
It was also a face palm because up until then I was sure that my dwarves, trained swimmers, would have no problems with getting out of water in case a wave swept them in. My entry is on the beach.
EDIT: Maybe if the ramps were there originally, he would get out. Or maybe not. Digging new ramps didn't help. To get rid of siege I have to wait to winter, and hope the besieger will be encased in ice. He doesn't even bother my fisherdwarves. Whole year of trade ruined, damn him.