The wall hexes on the sides of the protruding corners allow units to attack, but not to be attacked, in melee.

Not sure how to better describe this location, but units with pass wall or wall climbing can be in these hexes and attack units upon the wall. However, it is impossible to attack them in melee, or use touch abilities such as web, against them in that location.

This is an old bug, but I didn't see it on the bug list.