Minecraft wiki entry for Beginner%27s_guide, associated text: To protect a player, a shelter should be made out of blocks that cannot be walked or seen through by monsters. Holes in the walls or ceiling can be a safety hazard as baby zombies can fit through a one-block gap, skeletons can shoot through open gaps created by slabs or stairs, and spiders can climb over most walls. Creepers can blow up a house made of dirt or wood (stone is more durable), but only if they can see you. All that said, holes above head height (2 blocks or more above the ground outside) are out of reach for most monsters, and while spiders can climb, they cannot fit through a one-block wide gap. Fences (see below) can be used for windows; while monsters can see (or shoot) over a fence on the ground, if the fences are joined on all sides to blocks, the monsters can't see, move or shoot through them (and you cannot shoot through them either).