Pretty smart design.... Enough space to stick a sword through or aim an arrow at, but no more.
Open to the ground (I guess) so nobody could crouch down and pop up a surprise impaling while you're aiming that bow..?
Discussion
Idk about this particular place, but a lot of forts had "murder holes" (actual term), which were holes under these position where a dude could aim below or drop a big rock or something.