Back of sliding door and below wardrobe, also 2 other customizations:
I don't know if the bathroom door is solid core but it has a frame around it for screwing in the hooks. We don't use a bar.
Earl told us to use small screws with a dab of silicone for the t.p. holder.
The towel holder on the kitchen cabinet is a drawer pull screwed in from the back of the door. Should it need to be removed, the holes could be filled with wood filler. We also put one on the side of the bathroom sink for a handtowel. The over the door hooks were not good on those doors.
The coat rack is screwed in through the leather to the side of the cabinet. It gets a lot of use, jackets, leashes and lanyards etc.
We consider these things to be permanent, thus a strong attachment to the walls. For other things we use Command strips.