I have just completed my 3rd version of an XP Grinder. I created a 33x33 room, placing water along the North and South walls which run 7 blocks towards the mid-line of the room, dropping down to an 8 block run to the edge of a 3 wide trough running East-West. The trough has a 3x3 drop hole at the top of a 5x5 walled drop falling 20 blocks to my killing area.
1) I need to optimize the killing area to allow XP to flow out while the mobs don't. Short of placing an lever controlled iron door (ugly) in the walled enclosure the mobs fall into, I am getting frustrated (idea lock, brain dump, whatever...). Thoughts?
2) Is there any benefit to placing a second layer of spawners 3 spaces above the ones already placed? My spawners are set 2 blocks above the floor under them, allowing mobs to pass beneath them. I have a glass floor 2 blocks above the highest spawners (observation area), and 1 torch in each corner for minimal light. This area obviously would have to be moved up, but not sure if it's worth it. The basic set up is as follows:
The spawners are set up on a 5 blocks spacing around the center, using 32 spawners.