Also known as Principal (Great) Mosque of Damascus, founded by the Umayyad caliph al-Walid in 706 CE.
The Great Mosque stands in the centre of the old city of Damascus on the site of the Roman temple platform, or temenos. The outer walls of the temenos still survive and are distinguished as large blocks of dressed masonry with pilasters set at intervals into the side. At the four corners of the temenos there are large square towers and around the edge there were arcades which opened into a large rectangular courtyard. There were four axial doorways to the temenos, that on the east being the principal entrance. At the time of the Islamic conquest the Byzantine church of St John stood in the middle of this platform. Immediately after the conquest the Muslims shared this space with the Christians with the Christians retaining possession of their church and the Muslims using the southern arcades of the temenos as a prayer area.
In 706 al-Walid destroyed the church and built a mosque along the southern wall of the temenos. The layout of the mosque comprised three aisles running parallel to the south (qibla) wall cut in the centre by a raised perpendicular aisle or transept. At the south end of this transept there was a mihrab set into one of the blocked doors of the south facade. Walls were inserted on the west and east sides between the corner towers, and new two-storey arcades were built around the east, north and west sides of the courtyard. The arcades and prayer hall were covered with pitched wooden roofs covered with tiles except for the centre of the transept which had a wooden dome. In the north-west of the courtyard there is an octagonal chamber raised up on eight columns with a pool beneath. This structure functioned as the bayt al-mal or treasury and is found in other early mosques such as Harran and Hamma.
Since the Umayyad period the mosque has been rebuilt several times because of fires (1069, 1401 and 1893) although its basic plan has remained the same. Originally the arcade of the sanctuary facade comprised one pier alternating with two columns but this was subsequently changed to piers only. A range of different arch forms is used in the arcades including round, semi-circular horseshoe and slightly pointed arches. The walls of the mosque are decorated with glass mosaics similar to those in the Dome of the Rock, with depictions of palaces and houses next to a river (possibly the Barada river in Damascus). The long rooms in the east and west sides were lit by marble grilles with geometric interlace patterns based on octagons and circles.
The form of the mosque, particularly the sanctuary facade, was probably derived from Byzantine palatial architecture, possibly the Chalci palace in Constantinople. Later mosques in Syria such as the Great Mosques of Aleppo, Hamma, Harran and Cordoba. The Great Mosque of Diyarbakir built in the Seljuk period is also of this form.
K.A.C.Creswell (mosaics by Marguerite Van Berchem), Early Muslim Architecture, Oxford 1969, 1 (1): 156- 210, 323-72.
Detailed Article on Wikipedia: http://en.wikipedia.org/wiki/Umayyad_Mosque