Towers are needed to keep city walls safe and to keep a good lookout on dangerous roads. The arrow slits  provide great cover against enemy fire while having good lines of fire and strong walls keep infantry at bay. Throw rocks and oil on the heads of foes that try to break through the door.

Requirements: 4mm or 1/8 Inch thick material. The material used in the picture is MDF, but any material can be used.
Notes: This build does not require glue to assemble.