Size
Since you are buying it for your cat to enjoy itself, why not buy a big one so they can really stretch out. Don’t buy an outdoor cat house that is too small, because it defeats the purpose.
Fully Enclosed
Make sure the one that you buy is fully enclosed, especially on the bottom. As you know, cats are escape artists and will get out if it is not fully enclosed.
Security of Latch Zippers
Make sure if the doors are zippered shut that there is some sort of added security to them to prevent your cats from accidentally unzipping them. It has happed before!