Here's a rough sketch I started, based on the Dover Castle photograph. I began with some guidelines to suggest the main shapes and perspective, then roughed in the windows. To do the masonry, I used a 'broken line'approach, using some faint horizontal guidelines to keep rows of bricks or stones roughly lined up, then using broken vertical and horizontal lines to suggest the masonry. Vertical (rather than horizontal) shading is used to suggest the brick texture. You don't need to make it overly 'finished' and detailed when sketching - keep it fresh, and leave the imagination something to do.