Most mob grinders I've seen have a broad flat room for the monsters to spawn in. But I saw one recently that was a tall tower instead. That seemed more convenient. If you wanted to expand it you could just add another floor or two. But will it spawn as many monsters?
Does anyone know the rules that monsters spawn by? Do you get a limited number of monsters per x,z map area? Or will monsters spawn above each other in a large tower?
I know they don't spawn within about 24 of the player, but beyond that how are they distributed (along the x, z, and also y axes)?