Basilica A refers to a monumental church at the center of a larger complex of related buildings, including a mosque, located near the southeastern corner of the walled city of Rusafa (Roman Sergiopolis). The complex evolved over several centuries and was modified numerous times, but the oldest section is the church itself and probably dates to the late fifth century AD. The German archaeologist Thilo Ulbert, who published the first detailed study of the monument in 1986, originally identified it as the Church of the Holy Cross based on an inscription dated 559 AD.1 However, this identification was later revised by Ulbert and other scholars based on the fact that the form and architectural ornament dated the monument earlier, most likely to the end of the fifth century, and the fact that another inscription found on site stated that the shrine existed already in 518.2

The church takes the form of a basilica with a large central nave and two side aisles separated from the nave by an arcade. The central nave terminates on its east side in a semi-circular apse vaulted with a semidome while both side aisles terminate vaulted spaces to the north and south of the apse. The main entrance to the church is on the west side, through a narthex. Doors give onto both the nave and the two side aisles. The roof of the church may have been a pitched wooden roof forming a gable over the nave. 

The surviving fragments of architectural decoration in this church offers clues about what must have originally been a grander decoration program. Columns were adorned with carved capitals of the Corinthian order. A stylized stucco frieze consisting of pitcher-shaped blossoms emerging from crescent-moon forms adorned the apse. This stucco decoration bears notable resemblance to the most sculptural form of stucco ornament known from the Abbasid city of Samarra (Ernst Herzfeld's "Third Style") and was thus dated to a proposed ninth-century renovation by the site's early surveyors.3

When the Umayyad caliph Hisham ibn 'Abd al-Malik (r. 724-743) moved his residence to Rusafa, he constructed a hypostyle mosque on the north end of this courtyard, preserving most of the Byzantine structure and thus transforming the space into a multi-confessional complex.

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Sources:

Fowden, Elizabeth Key. The Barbarian Plain: Saing Sergius between Rome and Iran. Berkeley: University of California Press, 1999.

Guyer, Samuel. "Ruṣāfa." In Archäolodische Reise im Euphrat- und Tigris-Gebiet. Vol. 2., edited by Friedrich Sarre and Ernst Herzfeld, pp. 1-45. Berlin: D. Reimer, 1920.

Ulbert, Thilo. "Rusafa." Grove Art Online. Oxford Art Online. Oxford University Press, accessed April 20, 2017, http://www.oxfordartonline.com/subscriber/article/grove/art/T074511.