21.5000°N, 30.9670°E.
Middle Kingdom Egyptian fortress on the west bank of the Nile at the second cataract in what is now northern Sudan, built of mud-brick on a stone footing. Semna Fortress was constructed under Senusret III in the late nineteenth century BCE as the southernmost link in a chain of at least eleven cataract forts marking Egypt's frontier with Kush. The fortress occupied a granite outcrop that narrowed the river to roughly 40 metres, allowing complete control of upstream traffic; a stela of Senusret III erected on site forbids any Nubian boat or person from passing northward without authorization. The walls reached 8 to 10 metres in height and enclosed barracks, storerooms, and a small temple to the deified Senusret III. The site was excavated by George Reisner for the Harvard-Boston expedition in 1924 and again by Jean Vercoutter in the 1960s before being submerged by Lake Nasser.
The original border wall is now under fifty metres of water.