Those are really back-stabbing (or butt-stabbing, since they usually lurk below) guards.

They're posted in small guard-cells of the Castle de Los Muertos, and if someone passes by they ram their spears up through the ceiling to stop them. So.. never trust little holes in the ground, I guess.

Yep, that's right - you'll find them in the level Castle de Los Muertos. Areeba!