Purchased for daughters nasty little rats.  She loves them but they just creep me out.  I feel quite confident that they will never escape their new "condo", nor would they want to.  The rats really love going up and down through the levels.  I see the Critter Nation as a very healthy environment with plenty of good air flow.  My daughter loves the cage 1st, because her rats are extra happy and 2nd, because it's so easy to clean.  Even though the cage is enormous, it is quite easy to roll around and clean behind.  I agree with others in that a hammer was useful in putting the cage together, which was quite simple.  One thing I would like to see is a bolt or pin used on at least the opposite corners to secure the the top, hold the levels together and secure the base to the housing unit.  It is kind of top heavy, and it it ever tipped at least the animals would stay safe AND most of all, inside the cage.  This cage was a great buy.