(Sorry, my son and his friend won't let me have the castle to take a decent picture!)
smallish cardboard box - I used 11"x14"
4 cardboard tubes - we don't use many paper towels in our house, so I used some wax paper and such. I had to cut them all to the same length.
pen or pencil
With box closed, draw the top notches, windows and drawbridge door.
Using the exacto blade cut out the windows and the top of the drawbridge door. (Do not cut bottom of door - so the door will open out.)
Cut off the upper flaps of the box.
Remove all the tape on the body of the box. Leave the tape on the bottom. Glued boxes work the best.
With scissors, cut lengthwise along notches.
With exacto, score bottom edge of notch.
Fold notches in.
Take one of the removed lengthwise upper flap and cut two corners off along the same length.
Glue it along folded notches with notch underneath and cut corners at corners of the box. You can tape it in place to secure while glue dries.
Cut off remaining exposed folded notches.
Starting at the bottom of the cardboard tube, cut a lengthwise slit the height of the castle.
Cut a parallel line 1/4 turn away.
Cut notches on top, folding over or removing.
Repeat with other 3 tubes.
Slide the cut edges of the tubes over each corner of the castle.
Decorate as desired.