I was taking a look at some of the ways people have customized castes and keeps. Having seen one that was really nice but used a ton of sandstone rubble it got me thinking. While having different colors of stone is nice, I think adding in the different materials would be beneficial. As there is a lack of options available to those without chests full of rubble. My suggestion would be when mining sand have a chance for sandstone, and allow walls to be created out of that. This could apply for wood walls using logs as well. Have cooking kick in and bake the stone for brick walls. Even if you can't change the base layer of the castles or keeps this would expand the possibilities of what people have to choose from.