[mochaviolino]

I think when they say avoid multi-levels, its that some people end up building something like a two story 2x3 cage for four piggies. The dimensions on paper look like it makes up the proper sq ft, but taking out room for the ramps and such actually diminishes the square footage. They probably just want to make sure you provide adequate room for running laps and all that.

Personally, I think that as long as the "main floor" is the proper size then all's good. I usually try to count lofts and such as perks to their cage.