Simple mob farm and XP grinder. Made to use minimal cobble (half slabs) and other early game resources. Well suited to skyblock maps and building at Y-level 200. There is no way to toggle spawning. For that, I recommend upgrading to the redstone version, posted separately.
N.B. The spawn area is 2 blocks high. If you wish to include endermen then increase the ceiling height by 1 block, except above the half blocks in the centre of each of the 4 spawning pads, to avoid having mobs get stuck on them.