An important Islamic town in northern Libya which, in a region of semidesert, has one invaluable asset: water. Ajdabiyah therefore became an important caravan town at the junction of the main route from Egypt to the Maghreb and a trans-Saharan route from the Sudan. The town was taken by the Fatimids in their advance towards Egypt in 912, and flourished until it was destroyed by the Banu Hilal in 1051. Two monuments belonging to the period 912-1051 are important despite their poor state of preservation: an early congregational mosque and a qasr or fort. The mosque is roughly rectangular, with maximum dimensions of 47 by 31 metres. The courtyard has a single arcade on all four sides. The plan of the sanctuary is a simple T-shape, with a broad ‘nave’ and ‘transepts’ in front of the qibla wall. Stray finds include an inscription of 310 or 320 [ah] (922 or 932 ad). The qasr is a rectangular building, 33 metres long and 25 metres wide, with circular towers at the angles and a rectangular salient, including a monumental porch, at each side. The towers and salients appear to be for display rather than effective defence, and since the interior contains little more than reception rooms and magazines, the building has been identified as a rest-house for important travellers between Egypt and the Maghreb rather than a castle or palace.