I've read on some white cakes that they were sugar cubes, but what about other colored cakes? I will be doing a brown/tan castle -- how should I make these things??

Are you talking about the tops of the towers or the sides of the walls?
For the tops of the tower you ca roll out a strip of fondant(w Tylose or gumtex added) and cut out the square patter then wrap it around the top of the tower. If you could upload or link to a pic, you'll get more specific answer.



For the sides of the walls:
Will your cake be buttercream or fondant. Duff has a stone impression mat that would work for and old style castle wall. With fondant you could use a straight edge and hand impress the bricks, you could also cute out fondant squares.

Yes, the tops of towers & walls (not the actual walls - I do have an impression mat for those). What I"m talking about usually looks like miniature cement blocks along the top edges. Somewhere I thought I saw a pic of what looked like fondant that was just cut and wrapped/stuck onto the edge, and it did kind of lean forwards and backwards - so it didn't exactly carry off the "block" impression. I suppose I could make individual blocks from fondant and try to get them to stick.... it's not like this castle will be huge.