Minecraft wiki entry for Shelters, associated text: These are small holes in a wall that you can shoot arrows through but are small enough so that skeletons and other players have a hard time shooting back at you. This can be accomplished by stacking up cobblestone walls (the holes can be shot through) or simply digging a one-block hole through the wall and putting a slab on the bottom. Also, placing the non-block part of stairs facing each other, and using the upside-down stair function, you can make arrow slits almost impossible to shoot through unless you are close, so it is advised you put them far above ground.