San Stefano Rotondo is the only extant early Christian Church with a completely circular plan. Built to house the relics of St. Stephen, its form followed in the tradition of the ancient Roman mausoleums. Two concentric rings of columns surround a central octagonal altar and with the exterior wall make two annular ambulatories, the outer one of which includes the entry porch and a number of chapels. The columns could not carry a heavy dome and probably only supported a light conical roof. The central ring is lit by clerestory windows in a curving wall supported by the columns. A transverse arcade which bisects the center was later added to stabilize the structure and makes a striking spatial division.

— JY, GreatBuildings.com

Details

The inner ring of columns, which the transverse arcade bisects, has a diameter of 76 feet. The outer wall of the church has a diameter of 210 feet.

The exterior is brick masonry and the roof is framed in timber. The contemporary interior is plain white stucco, but there is evidence that it was originally faced in rich marbles.