I have a cat tower with about an A3 size footprint that is over 7 feet tall. That's for afternoon snoozing and plotting in the sun at the back of the house. And a cat barrel thing at the front window for snoozing and plotting in the morning. And a tall cat scratcher next to the settee so my overlord doesn't scratch the settee.
We bought tiny bookshelves and metal supports from B&Q and put them up the wall like steps to a doorway. The door has a much larger shelf across its top (4' long) using the wooden surround as part of its support, and a couple of larger matching metal supports. Hot glue gun to stick pieces of carpet tile on the shelves for cats to sit on (and use to stop them sliding off!). Works a treat. They love it.