Lay the first large box down horizontally on its longest side. Use the utility knife to remove the side that is facing up. Open the flaps on both small sides of the box so it becomes a long three-sided structure. Place a second large box at one open end of this structure so it overlaps the open flaps. Use the duct tape to secure the flaps of the horizontal box to the front, bottom and back of the vertical box. Repeat with the third box on the opposite end of the horizontal box. This creates the basic shape of the castle. The vertical boxes are the towers.
Cut out rectangular crenellations (attack points) along the top of the boxes as desired. Draw a door on each of the towers on the side facing into the horizontal box. These doors should not be taller than the height of the horizontal box. Cut out the doors to allow access between all of the castle boxes. Draw a door on the front of the horizontal box. Use scissors to poke one hole on each side of the door to create a drawbridge. Cut around the sides and top edges of the door and allow the drawbridge to fall open.
Measure two pieces of string long enough to go from the door to the wall of the castle. Tie a knot in one end of each piece of string. Thread the string through the holes on the door so that the knot is on the outside of the door. Use the scissors to poke a hole in the castle wall to either side of the drawbridge. Thread the strings through these holes. Pull the string to move up the drawbridge.
Decorate the castle. Paint the outside of the castle gray to represent stone. Use crayons or markers to decorate the inside with tapestries and designs.