I think the mosaic set is very useful, especially if you want to do something like this. You get a large gray baseplate and a border of 2x4 black slopes (all useful for the castle builder). The trick is to design your own mosaic so you get almost all dark gray, gray, and very light gray.
One problem--if you order a mosaic now, I'll bet it's all in new grays. Does anyone know about this?
The mosaic process itself is a tedious build, but has a very nice result. I made this as a gift for my SO (who collects Pooh):
http://www.ozbricks.net/bricktales/misc ... osaic.html