Minecraft wiki entry for Multiplayer_details, associated text: Non-operator players cannot place or destroy blocks in the protected area, nor can they use doors or Redstone devices other than Pressure Plates. Creepers can still destroy terrain in the protected area and TNT placed outside of the protected area will destroy surrounding blocks as usual. Along with this, portals from the nether can connect to the protected area, water/lava/fire can spread from outside into the protected area, and pistons can push items inside as well. Players can place blocks on the edge of the protected area if placed against the side of a block on the outside of the area. However, if the edges of the protected area are avoided, small, indestructible structures can be built. Paintings that are put up can be removed by players in the safe zone.