I started out with a very similar idea, and ended up making individual rock piles.
The hardest part is getting everything glued up and fitting together properly.
I'd you start by gluing two pieces of foam together to make the corner, texturizing it, and working off that piece. make pencil holes, and use a short pieces of wooden dowell and silicone everything to be sure of a strong bond.
After you've got the base made and shaped, get to work on the different levels using th same method to attach them.
Once you've got it all assembled, you can drylok it, then install it.
If you make the bottom level below the substrate, you can actually silicone some plants into it as well.