A way of making the best of a small space is to go up !

i.e. Tall cat trees and shelves/surfaces with additional cat beds etc. The more cat beds at different levels of your appartment, the happier your cats will be in a restricted space.

Also consider giving your cats safe limited outside access which can also increase cats happiness. Consider fencing in garden, building cat run, walking cats on leads & harness and fencing in balconies or fitting window seats.