Simple mob farm and XP grinder, upgraded from the early game version (posted separately). Uses redstone/glowstone/iron and replaces cobble with smooth stone. The redstone switch toggles spawning and uses sticky pistons to raise/lower spawning pads into water, effectively flushing any mobs out of the trap when it gets turned off.
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.