I took an oatmeal container and glued spitballs in roughly stonelike shapes. I attached them with a water/Elmer's glue mixture (basically paper mache).
Then I primed the whole thing gray.
Kind of sloppily drybrushed a lighter gray onto the surface of the stones.
It's a serviceable tower, but next time I'm going to prime it white and paint the stones gray (the lighter mortar will look more naturalistic) and I'm going to make those spitballs smaller and more regularly shaped.