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 Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.