Right, so this has been driving me a bit batty lately. I've got my "usual" fortress with ~100 dwarves and ~96 meat leeches, a moderately-sized surface structure, burgeoning pants and "shell-based tchotchkes" industries, a nice high wall all around outside, and one (count 'em) one 3-wide drawbridge, which has a 2-deep hole dug in front of it (granite-lined) to deal with the eventual arrival of "building destroyers". Two levels above ground I've got towers with fortifications all 'round (and another layer of roofing with it's own walls so that dead things can sun themselves into oblivion or "commercial viability" depending on their inclination) at the four corners, and elves-be-damned all the trees that overhang or even look like they're going to touch the walls cut down. To my understanding, this should be pretty safe. At least, it certainly used to be safe.
Invaders come, I sound the alert to get everyone indoors, throw the big switch and raise the drawbridge, send some marksdwarves up to the towers in the hopes they'll sober up long enough to shoot at some goblins, and...
A small number of invaders still magically get in, and a small number of besotted marksdwarves still find a way to get out and bumrush a much larger group of goblins and trolls (and afterwards are awarded their place in the Catacombs of Stupidity).
This has now happened multiple times in the eight years the fortress has been around, and I'm starting to get a little weary of it. It's basically every single time they come. The human werechameleon I could almost understand being able to climb the walls, but it's everything from goblin crossbowmen and archers to an entire freakin' troll.
What's going wrong here? Is it because I've made the slighly selfish choice of making the exterior structures out of chalk? There are definitely no gaps in the walls, and I put walls atop the second-level walls I'd carved fortifications into just so there'd be a "ceiling" and no one just climb over. These sneaky little buggers aren't apparently climbing up the outside walls... they're popping up right in the middle of things like they're able to teleport or perhaps throwing their buddies in with an invisible trebuchet.
Any suggestions as to how I can put a stop to this nonsense? It's putting a serious damper on my goals of creating a heavily-armored tourist trap and mercantile destination,