Minecraft wiki entry for Creating_a_challenge_map, associated text: Creepers are nice for dealing with your challenge map victims but can blow up parts of your structure, so try building the creeper room out of obsidian and add some inaccessible charged creeper spawners. Alternatively you can use /gamerule mobGriefing false to make it so that creepers, endermen, zombies, ghasts, withers, and ender dragons can't damage the environment.