Features
- Spawns Zombies, Skeletons, Creepers, Spiders and Witches
- Each layer has 348 spawning spaces
- Footprint of the design is 31 x 24 + a couple of torch towers either side

Rates
- After 1.5 hours of running with a 3 layer system on Normal difficulty:
  > Gunpowder: 3063
  > Arrows: 2923
  > Bones: 2940
  > Rotten Flesh: 2931
  > String: 2025
  > Witch Drops: 258 (Without Gunpowder)
  >  > Sticks: 77
  >  > Redstone: 30
  >  > Glowstone: 32
  >  > Spider Eyes: 38
  >  > Glass Bottles: 46
  >  > Sugar: 35

> Total: 14,140
> Rate: 9426 drops per hour

Comments
The design was for my Multiplayer Base which is next to an ice-spiked biome so I was happy to use lots of packed ice. It was also designed to be iron friendly so each layer uses under a stack of iron ingots (not counting the hoppers at the bottom).