Basilica B refers to a large church located toward the center of Rusafa (Roman Sergiopolis). The church is dated by a foundation inscription that was found re-used in the seventh-century Umayyad mosque built into the northern wall of the Basilica A Complex. The inscription states that it was constructed beginning in 518 AD in order to replace the older church and to house Saint Sergius' relics, which had been moved to another shrine in the city temporarily (most likely Basilica A located in the southeastern part of the city).1 Archaeological excavations confirmed that an older church dating no later than 425 AD lay underneath Basilica B, founded by Bishop Alexander of Hierapolis to house the relics of Saint Sergius. 

The church takes the form of a three-aisled basilica, with an apse at the eastern end of the nave flanked by two side rooms with which it communicated directly through doors. The room on the north side of the apse was a triconch and may have functioned as a baptistery. The church could be entered via a set of stairs on its west side or via a porch on its south side.

The most elaborately decorated space in the basilica was the triconch room north of the apse, which featured an opus sectile floor, glass mosaic vault decoration and painted architectural sculpture. 

Notes:

1. See synopsis in Fowden, Barbarian Plain, 84.

Sources:

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