Minecraft wiki entry for Defeating_a_dungeon, associated text: If you've found a zombie or spider dungeon, immediately block up the entrance with building blocks, and make a 1x1 hole in the dungeon's wall. If you can reach the spawner from the hole, place a torch or another light source on or near the spawner as soon as possible, so that less mobs will be spawning in. Use your sword to fight the zombies and spiders as they come. New groups of 1-4 zombies or spiders will spawn every 10-40 seconds, so when you've defeated all the current mobs in there, quickly break in and scatter torches everywhere to deter mob spawning. Don't look in the chests unless you're absolutely sure no mobs will spawn, as being ambushed while looking at a chest is difficult for you. Remember that all dungeons generate an entrance, so you could be attacked by creepers or skeletons from outside.