Cut the cardboard to the size you want for the project. Use the spray foam to attach sheets of Styrofoam to the cardboard. Build up the Styrofoam several inches thick and create a flat surface on which to build the castle in the middle. Use a marker to outline the perimeter of the castle.
Cut and shape pieces of Styrofoam to build the inside of the castle. Create various structures with roofs which will be connected to the walls once they're in place. Glue the pieces together to create the buildings but don't glue them into place yet.
Cut pieces of Styrofoam to create the walls and towers. Use the spray foam to connect pieces which you can carve once the glue has dried. Shape all the components first and position them to check the
fit. Complete all the shaping and glue the walls and towers into place.
Fill any cracks using the spray foam and let it dry. Glue all the buildings on the inside into place and connect all the pieces to the walls on the inside. Shape the spray foam with a knife or scraper so that the castle looks realistic, and use more spray foam and Styrofoam to fill in any gaps or holes.
Detail the walls, using a knife to create the lines in between the stones in the walls. Cut out openings along the top of the walls and some windows in the towers. Use your reference images to decide on the exact details as you carve the castle. Remove or shape the dried spray foam. Use a scraper to create the rounded shape of the towers.
Carve out a trench to indicate a moat outside the castle walls. Create the moat to hold water if you want to make it realistic. Carve the shape of a closed drawbridge against the wall of the castle. Create more structures outside the castle, such as a small building or a few hills using more Styrofoam. Finish detailing the entire castle.
Add features to the landscape outside the castle by gluing on more Styrofoam to create hills. Use the scraper tool to round the edges and the sandpaper to make them look smooth. Carve a road through the hills leading to the castle where the drawbridge is located. Add more buildings as desired using the same method as the one used to create the buildings on the inside of the castle.
Paint the castle with gray exterior latex paint. Use several coats of paint and fill all the crevices. Use some watered-down black acrylic paint and brush it on the top of the walls, letting it drip and soak into the wet gray paint. This will give a realistic look to the walls.
Add more colors to the surrounding area, including green for grass and a dark gray for the moat. Keep painting and add more colors to create a colorful landscape around the castle. Glue small pieces of sponge to toothpicks to create trees. Paint the sponge green and the toothpick brown. Simply stick them into the Styrofoam to secure them.
Let the paint dry for a day and then fill the moat with water to complete the project.