The oldest known castle was built by the Egyptians - Apparently.
The oldest known castle is the Red Pyramid's "castle" at Dahshur, Egypt, which was built around 2580 BC. This structure was not a traditional castle in the sense that it was a fortified residence, but rather a symbol of power and a fortified temple complex. The "castle" consisted of several stone buildings, including a 330-foot-long (101 meters) enclosure wall, and was built during the reign of Pharaoh Sneferu.