The Temple of Hercules Victor is a monopteros, a round temple of Greek 'peripteral' design completely encircled by a colonnade. Dating from the later 2nd century BC, and perhaps erected by L. Mummius Achaicus, conqueror of the Achaeans and destroyer of Corinth, the temple is 14.8 m in diameter and consists of a circular cella within a concentric ring of twenty Corinthian columns. These elements supported an architrave and roof, which have disappeared. The original wall of the cella, built of travertine and marble blocks, and nineteen of the originally twenty columns remain but the current tile roof was added later. Palladio's published reconstruction suggested a dome, though this was apparently erroneous. The temple is the earliest surviving marble building in Rome.
By 1132 the temple had been converted to a church, known as Santo Stefano alle Carozze. Additional restorations (and a fresco over the altar) were made in 1475. A plaque in the floor was dedicated by Sixtus IV. In the 17th century the church was rededicated to Santa Maria del Sole.References:
The Basilica of Santa Maria in Trastevere is one of the oldest churches of Rome. The basic floor plan and wall structure of the church date back to the 340s, and much of the structure to 1140-43. The first sanctuary was built in 221 and 227 by Pope Callixtus I and later completed by Pope Julius I.
The inscription on the episcopal throne states that this is the first church in Rome dedicated to Mary, mother of Jesus, although some claim that privilege belongs to the Basilica di Santa Maria Maggiore. A Christian house-church was founded here about 220 by Pope Saint Callixtus I (217-222) on the site of the Taberna meritoria, a refuge for retired soldiers. The area was made available for Christian use by Emperor Alexander Severus when he settled a dispute between the Christians and tavern-keepers.
The church underwent two restorations in the fifth and eighth centuries and in 1140-43 it was re-erected on its old foundations under Pope Innocent II.