I have this cage (two levels) and have 2 smallish adult girls in there.
I have only just read the post above (DOH) but agree wholeheartedly, the top area hardly counts as living space as it is basically a corridor leading to a sleeping compartment. I think it is probably only suitable for one fully grown guineapig if it is spending a lot of time in there. I don't worry too much about my girls, as they have a huge run for all day and in the winter their shed they can run around and play in, so they have lots of exercise. However, if they were just in this cage it would be insufficient. Another reason I've been very disappointed with this cage is the poor quality soft wood that it is made of, in many places it seems to be made of a kind of 'reformed light fibre wood' (I can't describe it!) Within minutes the girls had eaten holes through the ramp, infact you can use your own finger nail just to break bits off - very disappointing as a good hard wood would last much much better.