Measure the castle walls.
Cut four pieces of card stock. For each piece of card stock, the width should measure 5 inches; the length should equal the length of one of the castle walls. In this way, your four pieces of card stock surround the cardboard castle completely. Bevel the edges of the card stock out at a 45-degree angle, so that when you lay the pieces down around the castle, the sides of the moat pieces touch each other.
Make a mark 1 inch from the castle-wall edge of your card stock. Do this on both sides of the card stock. Draw a line, using your ruler, from one mark to the other, so that the line runs parallel to the castle wall.
Make a mark 1.5 inches from the castle-wall edge of your card stock. Do this on both sides of the card stock. Draw a line, using your ruler, from one mark to the other, so that the line runs parallel to the castle wall.
Make a mark 3.5 inches from the castle-wall edge of your card stock. Do this on both sides of the card stock. Draw a line, using your ruler, from one mark to the other, so that the line runs parallel to the castle wall.
Make a mark 4 inches from the castle-wall edge of your card stock. Do this on both sides of the card stock. Draw a line, using your ruler, from one mark to the other, so that the line runs parallel to the castle wall.
Fold along all of the lines on your card stock in the following order: at the 1-inch mark, fold down at a 90-degree angle; at the 1.5-inch inch mark, fold up at a 90-degree angle; at the 3.5-inch mark, fold up at a 90-degree angle; at the 4-inch mark, fold down at a 90-degree angle. This creates a basic moat form, where the bottom of the moat extends from your 1.5-inch mark to the 3.5-inch mark, with the walls of the moat dropping half an inch.
Color the moat appropriately. For example, you may wish to depict a moat without water, and color the bottom of the moat green and brown (to represent stone and moss). For water, color the bottom and sides of the moat blue and gray. For the sides of the moat, color to match your castle.
The moat at Spanish-run Fort Mose in Carolina was built out of clay, according to Historical Archaeology at the Florida Museum of Natural History. To replicate this moat, color the walls and floor brown-red.
Attach the wall of the moat to the castle using viscous glue, such as hot glue.