Minecraft wiki entry for Things_not_to_do, associated text: If a creeper sees a player next to such a hole, they can explode from the other side of the wall. The player can also use a collection of partially transparent blocks as windows such as wooden gates, stairs, slabs, or trapdoors as hostile mobs cannot see through them. If the players must have a hole and not a window, for instance, an arrow slit, a block such as stairs can be used. Two upside-down stairs facing toward each other in the wall leaves a space almost impossible to shoot through, making it safe from skeletons. A trapdoor also works great for this purpose, as the trapdoor can be flipped closed when the hole is not in use.