Nice! You really took my sketch and ran with it!
-South of the northern east gate there's a small glitch in the wall where two lines doesn't connect.
-It looks a bit like the north and east wall are below the slope of the ridge, I'd pull it back bit there
-If you use paths and stroke them instead of drawing freehand, the walls will look smoother.
-I like the size of the towers compared to the wall thickness
-A round bastion or barbican is stronger than a square one (they were square on the sketch to separate them from the wall towers)

I don't know much about PSP, but in PS you have something called blending options, with things like drop shadow, bevel and stroke. There's also a few overlays, color, gradient and pattern. I use pattern overlay with a stone texture and another stone texture on a thin stroke to get my walls.