If you happen to find a dungeon and don't destroy the monster spawner, you can create an XP trap. Why do you need XP? You can use your levels with your enchanting table — the higher your level, the better the enchantment. Ready to start farming monsters? Here's how to create an XP trap in Minecraft!
How to prepare
Assuming you've located a dungeon that spawns zombies or skeletons, it's time to prepare for the building process.
Place torches around the monster spawner and in the dungeon to keep mobs from spawning.
Empty out the chests, mine them until they break, and keep them in your inventory until later.
Outside the dungeon, create a safe-room with a door where you can place a crafting table, furnace, and the chests you collected from the dungeon.
Craft 23 signs.
Block off any open holes in the dungeon except for your access point. You can block this off later.
How to build a zombie or skeleton XP trap
One of the most effective XP traps you can create is one that uses a water elevator to haul monsters up, then drop them down to injure them. You can then finish them off with nothing more than your fist and collect the XP. This trap does not work for spiders, as they can stick to walls and usually won't let themselves fall to their death.
Here is a bisected, above-ground version of what you will carve underground. Notice the signs, the horizontal shaft at the top, the vertical drop at the end, and the opening at the bottom where you can hit the monsters' feet.
Mine the mossy cobblestone blocks that make up the floor of the dungeon so that there is a gap beneath the monster spawner.
Mine an opening that is two blocks high and one block wide in the middle of one wall of the dungeon.
Mine two blocks deeper into the wall.
Right-click with a sign in your hand to place the sign in the bottom corner opening you created.
Right-click with another sign one block above and one block to the right of the first sign.
Right-click with a bucket full of water in the open spot beside the top sign.
Mine the blocks above you. Make sure you alternate blocks so that nothing falls on your head and hurts you.
Right-click to place another sign one block above and to the left of the second sign you placed.
Right-click with a bucket full of water to place water in the open spot beside the sign.
Repeat this process of mining the blocks above you, placing a sign in an alternating manner, and placing water in the open areas. The signs keep the water from going anywhere, but won't stop monsters from being lifted up through the water.
To climb the shaft yourself to continue construction, just hold spacebar on your keyboard. Your character will not drown due to the open air pockets. Ideally, you want to place 23 signs in total in your vertical shaft. Any higher, and the monsters falling at the other end will die before you can hit them and collect XP.
The final sign you place should be on the far side of the wall from the dungeon.
Right-click with a bucket full of water both above the final sign and in the open area next to it.
Mine a horizontal shaft two blocks high and one block wide toward the dungeon that is below you — the result will be a shaft that goes over the dungeon. Mine this shaft so that it is eight blocks long.
At the eighth block, mine one block down.
Right-click the hole with a bucket full of water.
Mine another horizontal shaft that is two blocks high, one block wide, and eight blocks long.
At the end of the horizontal shaft, mine a vertical shaft about 21 blocks down. You don't want to dig straight down, so dig a 2x1 shaft, then fill in one side. There should be no water in this vertical shaft. If there is, the horizontal shaft at the top isn't long enough.
At the bottom of the vertical shaft, create a one-block opening. You will use this to hit the enemy mobs once they've fallen.
Return to the dungeon with two buckets of water. If you don't have enough iron, you can make trips back and forth from a water source.
Right-click with a bucket of water in each corner furthest from the elevator opening. This creates a flow of water toward the elevator that will funnel zombies or skeletons that spawn.
Break the torches on the monster spawner and watch out for any monsters that spawn! Wall up the dungeon, head over to the drop zone, and get ready to collect XP. Using your fist to hit monsters will work fine as they will be already heavily damaged from the fall.
Your XP farm
Have you had a chance to set something like this up on your own? What was the result? Tell us in the comments section below!