Minecraft wiki entry for Multiplayer_PvP_bases, associated text: 4. Anti-hybrid defenses- Even the secure base wall described above is crack-able. A cannon shooting both sand and TNT will allow the TNT to blow inside the sand block, and that causes damage. To counter that you can make upside down stairs, instead of a straight wall, with water going down all the way. That way the sand will fall down and the TNT will not blow.