2 years ago we adopted Nova & Naveen from a good friend who was fostering them in the Safford area. Nova's previous owner didn't get her spayed and she had multiple litters. We believe Naveen is one of her sons from her last litter. My friend fostered Nova until she weaned her last litter and then I paid to have her and Naveen fixed.
I built a matrix of shelves for them to climb up and down. I drilled them directly into the studs for extremely secure climbing. I also added a ceiling shelf from HyLoft to hold a large storage bin. This gray bin has a smaller bin inside it, stuffed with blankets where they love to take cozy naps together :)
On some of the shelves are patio chair cushions for comfy napping. I also keep their food and water elevated off of the floor. This helps with any ants that sneak in during the summer. I also firmly believe in allowing a cat to 'work' for their food, so a little jump on/off the table forces a little effort in order to eat. (I have strong opinions on allowing cats to demonstrate natural behaviors, which can help fight obesity in indoor cats.)
Nova & Naveen have all their nails, like nature intended and I added a wall mounted scratching post. I took a shelf, drilled it into the wall studs, then purchased a cheap floor rug, cut to size and stapled into the board. Staples are put in vertical to keep them from snagging a nail while they scratch.
On the other side of the garage is another area where they can climb up. I put up these shelves this summer and Nova & Naveen love to snuggle in the carrier, or lay on the cushion and look out the man-door.
The garage really is my work area... I have 2 freezers out there for our dog food, my mealworm bins where I breed my buggies for chicken food, and right now my seedlings are growing in there waiting for warmer weather to be transplanted in the garden.
Building 'kitty cities' isn't necessarily expensive, but it does take a bit of work. Most importantly, it's a great feeling to know you're offering your cats their own special place!