Place all three boxes in a row with the larger box in the middle and the two more narrow boxes on each side.
Tape the boxes together
Paint the boxes gray or black
Cut a large arch in the center bottom of the large box for the door. You cand cut it out completely, or leave the bottom attached much like a draw bridge would be
Draw or Cut two more arches for windows toward the top of the box. If you cut them out, cut them large enough for bean bags to fit through. If you cut them out, then save them to make gravestones for decoration
If needed, assemble the two small boxes
Using a piece of cardboard from the cardboard flaps, or scrap cardboard, cut out square open spaces to make the "battlement" ridge. This is the classic design you see at the top of castle towers with square cut-outs for soldiers to fight from.
Cut out an arch or arches for windows in each tower - you can keep the arch shapes to make into gravestones for decoration