Askut (also known in ancient Egypt as Djer-Setiu) was an ancient Egyptian island fortress in the Middle Kingdom on the Nile, which was built for the purpose of securing the border to Nubia. Since the completion of the Aswan High Dam, the island has been flooded with Lake Nubia.
The fort, about 351 kilometers south of Aswan was built by Sesostris III. It measured 77 × 87 meters. The protective wall had a thickness of 5.3 meters and had spur-like bastions. The highly fortified entrance protected a temple and warehouses along the harbor. Inside the castle were a commander's house and barracks. Pottery has been unearthed at the site dated to the early 13th Dynasty.