Pegasus produce a range of prepainted plastic 28mm wargaming scenery; all the stuff I’ve seen has been well cast but mostly badly painted. The stone wall sections are good value for money, though, with six 6″ sections for $10 at my Friendly Local Gaming Store.
I picked up a pack, took them home and while repainting them, took enough photos to assemble into a quick, hopefully inspirational, how-to.
(Click the image to see the full-size version at Flickr)