I review enclosures, providing honest feedback on good enclosures and improper cages. A mouse is inside temporarily as I review this but he returns to his Preview 528 shortly. I'm comparing this to my WARE topper. Now to the review: This cage is fine for a mouse provided you fill with bedding and add toys, ropes, tunnels, hammocks, houses, chews and plenty of out-of-the-cage time for mental stimulation. However, plastic is bad for your pet's health if they chew on it long-term. The balconies are extremely small - too small for toys, wheels or houses, just large enough for the tiniest chews. The bars are okay, if not a bit flimsy. This enclosure is fine for a mouse. Hamsters need 450 sq in (undivided floor space), gerbils need 360 sq in and mice need a minimum of 200 sq in - a 10 gallon or larger. Because this fits atop a 10 gal it's technically suitable for 1 mouse but, if I remember correctly, it has a bar spacing of 1/2" which is only safe for large male mice. Something to consider; wheel size, another reason hamsters and gerbils aren't suitable. The smallest hamster, the roborovski, requires a 6.5" wheel and most dwarves (winter white djungarian, campbell's russian, hybrids and chinese) require at least an 8" wheel. Syrians need a 9-12", gerbils need an 8-10" and mice need a 7.5-9" wheel. The cage can house a 9" wheel at most but that takes up a lot of space and the ledges are too small anyway. Overall, I prefer the original Petco topper. Although the curved top takes up space, it's sturdier and the latch is more secure. It costs more initially unless you get it on sale (I suggest you do), but it's better overall and doesn't have dangerous plastic. It's the (WARE?) OG Petco topper for me!