Vaulted arcade, The Great Umayyed Mosque of Damascus, Syria

The Umayyad Mosque, also known as the Great Mosque of Damascus (Arabic transliteration: Ğām' Banī 'Umayya al-Kabīr) or formerly the Basilica of Saint John the Baptist (Greek transliteration: Vasilikí tou Agíou Ioánni tou Vaptistí), located within the circuit walls of 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 incorporated into the Christian Basilica dedicated to John the Baptist (Yahya). The mosque holds a shrine which today may still contain the head of John the Baptist, honored as a prophet by both Christians and Muslims alike, and is believed to be the place where Isa (Jesus) will return at the End of Days. The tomb of Saladin stands in a small garden adjoining the north wall of the mosque.
Pre-Islamic period
Damascus was the capital of the Aramaean state Aram-Damascus during the Iron Age. The Arameans of western Syria followed the cult of Hadad-Ramman, the god of thunderstorms and rain, and erected a temple dedicated to him at the site of the present-day Umayyad Mosque. It is not known exactly how the temple looked, but it is believed to have followed the traditional Semitic-Canaanite architectural form, resembling the Temple of Jerusalem. The site likely consisted of a walled courtyard, a small chamber for worship, and a tower-like structure typically symbolizing the "high place" of storm gods, in this case Hadad. One stone remains from the Aramaean temple, dated to the rule of King Hazael, has survived and is currently on display in the National Museum of Damascus.
The Temple of Hadad-Ramman continued to serve a central role in the city and when the Romans conquered Damascus in 64 CE they assimilated Hadad with their own god of thunder, Jupiter. Thus, they engaged in a project to reconfigure and expand the temple under the direction of Damascus-born architect Apollodorus who created and executed its design. The symmetry and dimensions of the new Greco-Roman temple impressed the local population. With the exception of the much increased scale of the building, most of its original Semitic design was preserved; the walled courtyard was largely left intact. In the center of the courtyard stood the cella, an image of the god which followers would honor. There was one tower at each of courtyard's four corners. The towers were used for rituals in line with ancient Semitic religious traditions where sacrifices w