The interior is laid out like a cloister, with arched and covered colonnades around a central area.

It was begun by the Emperor Maxentius (reigned AD 306-312), possibly following the fire of AD 307, but was only completed, in slightly altered form, after his death in the Battle of the Milvian Bridge (AD 312). The Senate subsequently dedicated it to his victorious rival Constantine. The collapse of the nave and south aisle in the medieval period created the imposing ruin visible today. It was a source of spolia for other buildings; the last remaining column was removed by Pope Paul V in 1613 to its present location in front of S Maria Maggiore in Rome.