I pulled some options that may be of interest to you below. I am thinking to stay away from anything that is horizontal unless it is enclosed as not to become a ladder. I was also thinking but could not fins a picture, what about a smooth stone wall with vertical metal posts at the very top for interest.

Love the idea of this being security and raised garden with seating for entertainment. Change the top slats to be vertical.

Understated backyard

Simple elegant sealed

Love the top detail and wide wood

You could always do what the original Spanish land grantees in South Texas did to secure their ranch compounds. They built their houses out of sillar or limestone blocks and embedded glass shards in a layer of cement on top of the walls. Another option: plant cactus in a planter on top of the wall ocotillo or coachwhip is pretty nasty to deal with.