Minecraft wiki entry for Wither_cage, associated text: The idea is to trap a wither in an inescapable room. Withers can shoot skulls, which break certain blocks. If they are damaged in any way, they can break nearly all blocks around them. No matter the cage, the wither can escape if it is not built correctly, or if the chunks reload and inaccurately place the wither. One small change in positioning can end up with a wither on the loose, which could wreak havoc on its surroundings.