Minecraft wiki entry for Defense, associated text: Instead of a door, a useful trick is to make a 3 block high entrance, then cover the top and the bottom with any sort of slab. The mob's navigation system tells them that they cannot go through the opening. Beware though, as skeletons can still shoot you and creepers can still explode. Planting vines in front of the entrance will cut their vision. Bamboo can also do the same thing as slabs, while skeletons can still shoot through.