I had bought a cat tree off eBay a couple years ago --- it had 2
shelves and a top shelf on a 5 ft pole (pole was a 4x4 square). Well
it wasn't very well made and fatty Alfred broke the shelves on day one
but the carpeted pole and top were still intact. It became the
ultimate in king of the hill. whoever made it to the top could easily
repel any others by just slapping at them as they tried to climb up so
they ruled the top.

My one kitty Xena has an ongoing health problem (autoimmune hemolytic
anemia) and sometimes she just doesn't feel like playing and the boys
(there are 3 of them) can get a little too rough for her sometimes.
She would go to the top of that tree and no one could bother her
there.

So I suggest a tree or shelf on the wall with only 1 way up. You could
make an angled pole or shelf to get up to it if a straight up were too
difficult for your kitty. Mine would have a little more trouble
climbing it in the days right after having their claws trimmed.

Mel