Well, it certainly took a while, but a couple of weeks ago, I added a new surface - much steeper and of different material. After seeing the gaps that opened up on the first wall when I used 5/4 x 6 decking, I opted to try some treated plywood. This wall is installed at a much steeper angle. After securing the wood, I let it sit for a couple of weeks. As it dried out or aged or whatever, I noticed that it became rather rough and splintery. I certainly didn't want the girls to be brushing up against that. So, a trip out with the power sander and we're all good. The girls helped me to select and connect the holds and now they have a much more advanced wall to scale.
Up next, the cargo net!