Look For Mob Spawners

 

Enemy spawners can be extremely frustrating. They keep spawning enemies into the vicinity until you get tired or are defeated. Unless of course, you are able to destroy them. But before that, there is another step.

Just place torches all around the spawner. This is a better idea than destroying it. Torches will prevent enemies from spawning. Also, if you need to farm a certain type of enemy, you can just remove the torches to let them spawn.