@ossobuco,
Even when it's ok to begin with, they do their thing (scratching) and it looks like ****.
Best I ever saw was a built in one.....the tower hugged the wall up to the ceiling, and there were platforms on the way up...it hugged a door that was the one leading to the utility areas of the house, and there was no door in that doorway....it was at the back, looking out to thr garden, and the carpeting was the same colour as the walls. The cats were brown Burmese, so the platforms kind of disappeared and they stood out.
The owner simply resigned herself to having the carpeting replaced fairly frequently. The cats adored it...and it was the least intrusive of those things I have ever seen.