Minecraft wiki entry for Defense, associated text: Creepers have an explosion nearly as powerful as TNT, so they can easily blow through a wall made of stone; if this happens, it can break a hole in the wall, so that other uninvited mobs can enter your protected area. The only true wall that will protect against creepers is a wall made of bedrock or obsidian, although the former can only be used on a multiplayer server with item spawning enabled, with a mod, or via creative mode. A self-repairing wall can also be created using a cobblestone generator. Place lava and air alternating and also place waters diagonal to the lava. Note that you have to have the lava as source blocks. See Tutorials/Cobblestone farming for more information about different designs of cobblestone generators.