Not quite as drastic, but I moved my cats from 2200 sq ft down to just over 1,000 in a condo and I made sure to give them window seats and cat trees to go up. I've seen some cool shelves for the wall and trees that are narrow but go all the way up to the ceiling for smaller spaces. One of them has gotten a little chunky from not being able to run as far, but I'm using the laser pointer to get her to run around a lot more now. I'll lead her up on the bed, across the room and in circles to get her going. Also added some butterfly attracting plants on the balcony so they have a lot to look at.