Semna was a fortified town established in the reign of Senusret I (1965-1920 BC) on the west bank of the Nile at the southern end of a series of fortresses founded during the 12th Dynasty (1985-1795 BC) in the second-cataract area of Lower Nubia. The Semna gorge, at the southern edge of ancient Egypt, was the narrowest part of the Nile valley. It was here, at this strategic location, that the 12th Dynasty pharaohs built a cluster of four mud-brick fortresses: Semna, Kumma, Semna South and Uronarti (all covered by the waters of Lake Nasser since the completion of the Aswan High Dam in 1971). The rectangular Kumma fortress, the L-shaped Semna fortress (on the opposite bank) and the smaller square fortress of Semna South were each investigated by the American archaeologist George Reisner in 1924 and 1928. Semna and Kumma also included the remains of temples, houses and cemeteries dating to the New Kingdom (1550-1069 BC), which would have been roughly contemporary with such lower Nubian towns as Amara West and Sesebisudla, when the second cataract region had become part of an Egyptian 'empire', rather than simply a frontier zone.

[http://en.wikipedia.org/wiki/Semna]