The monster AI in Doom causes attacking enemies to reverse direction and feint when the direct route to their target is obstructed. This normally adds to the realism and difficulty of combat (the cyberdemon on E2M8: Tower of Babel, for instance, could otherwise be killed through a sniper hole), but not always. Any mobile monster at all, having repeatedly crashed into a wall or other monsters in its attempts to reach the player, eventually turns around and runs away from the player, sometimes reaching the opposite wall of the room before deciding to re-engage.