Not much to say about how I built it.
I cut a lot of stones in about 4-5 different sizes from Styrofoam. When that's done you have to squezze them in your hands to make them look older and not as regular as thez come from the cutter. Then I built it with whiteglue as mortar. When it was dry I applied a coat of black colour, sand ans woodglue mostly where have been gaps between the stones. When that was dried, just spraypaint in black and a second soft layer in white and paint the single stones. I mixed my stonecolour mainly from saddlebrown and red, and some ochre, Brown, white and pink for the different stones. At the end a simple drybrush with a mix of the main colour and white over the whole building makes it come together.