Ok, this step is pretty hardcore. It's also optional. You'll notice the main lines of the tower are pretty bold and sterile. If you're on the computer, use the Eraser tool next. If you're doing this by hand, you might want to use a white gel pen or any other white paint/ink/gesso/gouache/whatever. Go in and start eating away at those bold lines with your brush. Leave the outer lines mostly intact and focus on the inner edge of those lines. Keep your marks ragged and feel free to tap around and be random. This step will transform your drawing from a collection of flat shapes to a detailed-looking, highly-rendered illustration. I could keep working on sections of this image for countless hours, but I'm going to step back, slap some color on it, and call it a day. Hope you all enjoyed!