Minecraft wiki entry for Spawn_jail, associated text: If there is a room with bedrock as a ceiling, it would also be better to put the spawn jail at the top. When a player respawns and there is a block on top of the spawning point, that player will be transported over the top of the map, trapping him, and will have to rejoin. Unfortunately, if the player places the block at the spawn block, anyone else who joins will be stuck. Also, note that this method would only work with cave maps or closed underwater maps. The jail buildings with iron bars are optional.