This is not your traditional XP farm. First I didn’t want to use a “spawner”. I understand that this is the new and easy way to generate mobs but it felt like cheating to me. Further there wasn’t one where I wanted it to be so the point was kind of moot. I also did not want some huge thing floating in the sky ruining my views.
The Spawning Pads
In a large area that I flattened I dug 24 blocks down. This is the location of the spawning pads. This distance is important as mobs will not spawn closer than 24 blocks to a player, further after about 36 blocks or thereabouts mobs may stop moving. In this design we do not have a mechanism to push mobs into the trap and rely and them moving around on their own.
From that square I dug out an area 49×49, 2 blocks high. In retrospect this was bigger than it needed to be. I then built a second floor above that to same specifications. Embedded in the floor of the upper layer are Redstone Lamps. The redstone wire comes up to the surface allowing you with a lever turn the lights on and off. Some of the open area on the pads are also covered with half-blocks such there are no (or very few) areas where spiders can spawn. The pads have 1 block-wide channels dug in every 8 blocks. The channels have water to push mobs to the larger main channels (3 blocks wide). The signs on the side of the blocks trick the AI for the mobs into thinking that the space is not empty. This way they walk off the edge and fall into the water.
The Spider Trap
While most of these mobs are not spiders we still might get a few. Spiders are hard to deal with and I do not want them clogging up the XP farm. The four wide water channels by the spawn pads all push to a center hole that is 3×3. Once mobs fall into this hole they again land in water. This water pushes towards a lava blade but there is a one block hole before it can be reached. This lets one block footprint mobs to fall down while allowing the spiders to “bridge” across to the water on the other side. No more spiders. If you are interested, the spiders drops will fall (if not burned up in the lava) and can be collected separately.
When mobs are touching water they will attempt to swim to the surface. This is the technique we use to bring the captured mobs to the surface where we will kill them. However, if we created just a column of water the mobs would drown. With the creative use of signs and a LOT of water source blocks you can create an elevator they will swim up but not drown
The Kill Box
The elevator goes all the way up to the surface and continues about 22 blocks. We don’t just want the mobs trapped for us to kill we want them weakened and bloody, just barely alive so that we can kill them with one or two hits of anything (like bloody fists!). Turns out a 21-22 block fall is just about right. Additionally, there is a pressure plate to our right which will push fallen loot to us so that we can reach it.