Urist McUseless cancels eat: interrupted by forgotten beast. er.. that's odd. Zoom to location.. wait, that forgotten beast is in my main stairwell. Outside the dining hall, which has ~40 dwarves in it. A giant cockroach with lidless eyes! Beware its deadly blood! Deadly blood. Outside the most heavily trafficked area of my fort. Well, it was a fun fort...
I tell my military to attack. Turns out one marksdwarf recruit was in the dining hall. He walks 3 steps to the door of the hall and fires a single shot into the beast's head, killing it instantly. And more importantly - without spilling a single drop of blood. Oh yeah, someone's getting all masterwork furniture and a tomb...
So now, my question is - the hell did it come from? I built a single 2x2 stairway down to the magma sea. The magma is blocked off with the impassable tiles of workshops. I only hit the caverns in two places. In one, I put up a wall to seal it off. The other is the only place I can guess it came in at. One of the 2x2 stairs pierced the cavern. I floored over it and moved the stairway over a few squares:
X = up/down stairs
. = cavern floor
F = constructed floor
W = wall
z-0
WWWW
WFXW
WXXW
WWWW
x-1
WWWW
.XWW
.WWW
WWWW
Could an enemy path up through those stairs even with a floor constructed over the z-level above?
I've had one siege where a group of trolls spawned inside my underground tree farm (1 z under the surface), so I know enemies sometimes just appear where they shouldn't. Maybe this is another case.