Shelving on the wall is a great climbing tool for cats! They love it. Actually, a super highway (so to speak) is recommended by Jackson Galaxy. They need an entrance and an exit and the shelves need to be wide enough to accommodate Tiger sleeping on them. Bengals LOVE being high up in the air. The only key is that you live in an apartment and the landlord may not want holes in the wall. In that case, you can buy cubes and just stack them on top of each other and create a step effect.
There are many companies out there that make insulated cat houses. The one I designed is really huge since I had a very large cat using it. I don't even remember how much it cost us, but it's very heavy. My husband loves woodworking and creating things -- but doesn't have much time since he works full time and plays golf the rest of the time.
Maybe when he retires .... nope, then he'll play golf ALL the time.