Syria: the dome of the clock at the Great Mosque of Damascus, the Umayyad Mosque. The Dome of the Clock was built in 780. The Umayyad Mosque, also known as the Great Mosque of Damascus located in the old city of Damascus, is one of the largest and oldest mosques in the world. It is considered the fourth-holiest place in Islam. After the Arab conquest of Damascus in 634, the mosque was built on the site of a Christian basilica dedicated to John the Baptist.