World Download Available Here
Update 1.5.17: Fixed Minecart elevator.

What is this thing?
This is my AFKable gold and XP farm which I built in my survival world. It's based on a design by Snocrash which went quite viral in the technical world of Minecraft. I've made some improvements to it myself to make it more efficient, user-friendly and, most of all, safer to use. I called it 'The Circle' because... it's a circle! From the outside anyway. I'm no good at naming things.

How does it work?
To start farming, first, you have to turn on a mechanism which pushes the XP to the player standing on the collection platform and the items to the storage system. This is done with pistons, slime blocks, and ice. Then you have to make the Zombie Pigman angry so that they want to kill you, to do that you shoot a pigman with a bow (or a... snowball... if you want...), this will automatically make all of the surrounding pigman angry at you as well. They will then try to pathfind their way to the player via some hatches, but what they don't know is that this actually lures them into a drop enough to kill them, and so we get their XP - the reason why they drop XP now is because the player is the one who harmed them first.

Then, the items and XP get sent on their way to their right place. The items are picked up by hopper minecarts underneath the ice, then they are sorted so that only gold nuggets and ingots end up in the storage areas. All of the other items (rotten flesh, gold swords, etc.) get dispensed into some lava.

So is it fast? Yes, very. For collecting XP, this is probably the fastest farm out there. You'll get up to level 200 within an hour or so. However, you have to remember: The higher your XP level is, the longer it takes to get to the next level, so it's best when you don't AFK too long at a time if you don't want to end up with a gold overdose. Certainly, you don't need to worry about gold ever again.

The build
It took a while. About 3-4 months in total, designing, building, and testing, with some time gaps in between as I also went on holiday and such while I was working on this. What was also challenging is that this thing is built midair way up on the height limit which meant breaking through the bedrock ceiling of the Nether (I used a glitch to do it which has been patched since) before anything could happen in the first place, and while building I died plenty of times from falling off, getting rekt by angry Pigmen, Magma Cubes, Ghasts and even an Enderman once,

It's not the best looking building in the world, but it is what it is. Sometimes function is more important than looks. I had to be very careful to make sure that the mobs would only spawn on the spawning platforms and not outside of it, so all of the floors are either half slabs, stairs or are covered with a button or something else that prevents mobs from spawning where they shouldn't be. Lightning doesn't matter here because Nether mobs spawn at any light level.