I built these ruined walls from aquarium rocks and scrap mdf pieces. They're not the tallest ruined walls, but with the added height of the mdf they do the job.
Here are the rocks used
The unpictured steps are my standard basing/painting techniques; add sand, prime black, drybrush the rock walls in a medium grey followed by a lighter grey drybrush, paint the sand/dirt dark brown, and drybrush a grey/tam. Flock and foliage were then added.
This piece uses a flatter aquarium rock, so they resemble slate. The base is cardboard, covered with hot glue to give texture as well as strength. Some sand was added in various spots because the hot glue texture looks a little off by itself. Hopefully this will look like some cool ruined ground.