Minecraft wiki entry for Shelters, associated text: The first thing you must do is find a spawner. Then seal off any extra entrances and place a door at the last remaining one. Maybe make the dungeon a little bigger, to provide for more fun and monster spawns. Finally, make a lobby with chests full of equipment (swords, bows and arrows), various helpful potions, and food. Now you can fight that specific mob whether for experience or drops for as long as you want! Include beds if it is a long way away from your base, so that if you die you won't have to walk for ages just to find it again, or simply make a minecart track to take you there quickly. If in multiplayer, add a lock on the lobby and another lobby where other players can pay you to fight that mob and rent equipment and various supplies. This is handy to build in abandoned mineshafts, wherever you find a cave spider spawner, but if you do that make sure you have plenty of milk buckets on hand. If you want to give your arena a bit of a twist you could add challenges, like a room where the mobs spawn over your head or one where you have to solve a puzzle while fighting the monsters.