It is hard to see the mesh on top of the walls so have posted a pic of the corner. The mesh is attached by metal brackets which angle in. The cats would have to get on top of the wall and then go backwards, upside down, then over the top of the mesh. And the mesh is flexible and they don't like it underfoot as they wouldn't feel stable.
What they do is get up on top of the wall, with the mesh over their heads, and watch my neighbours when they are in their gardens. At first one would patrol the walls as if he were Steve McQueen in the Great Escape, just making sure that there had been no breach in the perimeter. Now they don't bother. We have had no escapees so far (except when hubby was first here and managed to let one out of the back door).
If you keep a cat indoors exclusively it soon adapts and settles down.
I couldn't cope with having 7 of them indoors all the time so this was a good option.