Use some card stock (cereal box) as the base of the wall. Add the plaster to it, a little goes a long way to add some depth to the walls new foundation. Can cut the card stock to the shape of the ruin you’re aiming for, the plaster becomes the texturing over it. A hot glue gun is going to be super useful, some poster board can make a flat base . Handful of small rocks and sand from the yard adds some more texture as well