Hold your pieces of cardboard together and check whether they are the same length and width. If they are not, trim them until they are.
Place one of your cardboard tubes vertically on a piece of paper so it is standing up on one of its ends.
Trace around the bottom of the tube with a pencil.
Use your ruler to draw a square on your paper inside the circle so that all four of its corners touch the inside of the circle.
Measure the length of the sides of this square. Write this measurement down.
Place your ruler along an end of the cardboard cylinder so that it partially covers that end.
Move your ruler until the distance between the sides of your cylinder matches your the length of your square. Mark these spots with an "x." Repeat this step with your other cylinder.
Draw a line from the marks on a cylinder down to its bottom. Repeat this with the other cylinder. Both cylinders should now have two lines running down them.
Subtract the length of one side of the square on the paper from the height of your cardboard while it is set up like a wall. Write this new number down, it is the height of your walls.
Make an "x" on both ends of one face of your pieces of cardboard at the measurement of the height of your walls. Draw a horizontal line to connect both of the "x" marks.
Draw a level line with this point across the face of each piece of your cardboard.
Measure the length of your pieces of cardboard. Write this number down.
Divide the length of your cardboard pieces by three to find the length of your walls on either side of the drawbridge.
Measure and mark the spot that is at this distance from either end of both of your pieces of cardboard. Draw an arch joining both points on each piece of cardboard.
Cut your cardboard pieces along the line drawn on them.
Clip your cylinders along the lines on them from their bottom up to the height of your larger pieces of cardboard (when they are standing on their sides).
Cut your arches out, leaving enough space at the bottom of both to keep them attached to the walls. Cut one arch out entirely and fold the other outwards.
Make holes by sticking your pin in two places over the hole left by the arch door, and in two places near the end of the bridge itself.
Thread a piece of string through each hole in the bridge and up through each hole above the archway.
Tie knots at the bottom of each piece of string to hold it in place below the bridge, and tie knots in the tops of each piece of string to hold them in place over the archway.
Cut a slit 1/2 inch below the top of your pieces of cardboard.
Slide your pieces of cardboard into the slits running up your cylinders.
Slip one of your smaller pieces of cardboard into the slits in your walls to make a walkway along the top of your wall.
Cut your remaining small piece of cardboard into smaller strips so that it can be laid between your walls and serve as a walkway through your archway.