First, we have a Mob (Enemy) Grinder. This is a build that lets you use enemies to get lots of items and experience. 

You will need:

25 full stacks of cobblestone

One full stack of trapdoors

Four hoppers

Four chests

Four slabs of any material

Two buckets of water

Now that you have all of these, it’s time to build.

Let’s get to the steps on building it. 

• Dig a 2x2 hole 1 block deep.

• Place your chests inside of the hole making sure they are facing the same direction.

• Place your hoppers on the top of the chests.

• Place slabs on the top of the hoppers.

• Place 2 cobblestone blocks 1 block above and to the side of the slabs, then raise this by 22 blocks in the sky.

• When raised by 22 blocks, build out on each side by 8 blocks.

• Now build 2 block high walls on each side around the platforms.

• After that, build out on the 2 block high walls so there is a platform on each corner.

• Finally build 2 block high walls on the edge of the platforms and fill out the roof so you have a dark room (mobs might start spawning so be careful).

Now that you have the platforms built, It's time to get the mobs to fall in the hole.

• Place water at the end of the gaps between the platforms so that water goes just to the edge of the hole (making sure the water does not fall in, if the water goes in the hole then a step has not been followed correctly).

• After this, place trapdoors on the top of the gaps so that they are just above the water and not hovering in the middle.

After this your mob farm should be complete and you can wait at the bottom of the farm (on the outside) for the enemies to spawn.