The fort at the second tier is made from a piece of pink foam and clad in corkboard. Another more grainy cork mat is used to create the barracks. I'm sort of out of white so what I mostly use is sand mixed with black.
I took my BB gun and shot a bunch of holes in the ruined keep. The only problem was that is splintered the balsa wood floor. Next time I do this I'll have to make separate floorboard which can be torn up individually.