A simple iron farm

 The world's simplest iron farm. The hardest part about this farm is getting the villagers up but you can do that with a simple rail line 

Step 1

Pick your spot you want to build and make a 2x2 tube at least 20 blocks off the ground

Step 2 

Build a 18x18 platform on top leaving the center open for the golems to fall through

Step 3 

around the platform with walls or fences

In the corners add some walls on the second lever to contain the water

Step 4 

Time for water in the corners you will want to put blocks to keep from filling the whole thing

Step 5 

4 blocks up over the center you want to either make a 5x2 or 6x2 platform for the villagers with 5 or 6 beds depending and encase it in glass to keep them safe

Step 6

The killing and collection of the golems at the bottom of the drop tube place signs on the second block and lava above them to kill the golems 

Step 7

4 hoppers in to a chest and you are done

This is not the most efficient iron farm but it is cheap and you can expand it with more platforms that drop to lower ones.