Use a felt-tipped pen to draw the ruined wall section on a piece of styrene insulation foam. Draw on all the rough details as well, such as stone placement, arrow slits, broken masonry, and arched doorways. This sort of pre-planning will pay off with a headache-free detailing stage.

Note: Measurements are shown in the picture so that you can try this piece first as a "dry run" if you are so inclined.

Cut out the basic form with a retractable, breakaway box cutter. Stick to your inked guides, and you can't go far wrong.

Cut a 45-degree corner out of the side edge of the wall to serve as a join when assembling the ruined wall section (so two wall sections will form a 90-degree corner). Repeat this step on the adjoining wall section.

Note: For this example, I've just created one section of the wall for demonstration purposes. When you build your own ruined castle, you should glue your walls together into a corner before moving onto the next stage.