Since we want the brick walls to have a natural, worn-down look, we'll add some special effects to make them look less bland.

These castle walls are supposed to be quite old, so let’s add some mold!

We’ll use a very dry brush and randomly add some greens here and there. The greens should be stronger at the bottom of the walls and get weaker at the top.

But this is still not enough!

Let’s use our opaque white to add some other old age symptoms!
With dots, circles and random strokes we can get the walls to look a lot more realistic.