Caracalla, in 215, introduces the antoninianus, a "double denarius" weighing 5.1 grams and containing 2.6 grams of silver—a purity of fifty-two percent.

A temple built on Quirinal Hill and dedicated to Serapis was, by most surviving accounts, the most sumptuous and architectonically ambitious of those built on the hill; its remains are still visible between Palazzo Colonna and the Pontifical Gregorian University.

This is one of a number of colossal temples built by the Severan emperors and dedicated to exotic foreign deities.

The sanctuary, which lies between today's piazza della Pilotta and the large square facing Quirinal Palace, is built by Caracalla on the western slopes of the hill, covering over thirteen thousand square meters (3.2 acres), as its sides measured one hundred and thirty-five meters by ninety-eight meters.

It is composed by a long courtyard (surrounded by a colonnade) and by the ritual area, where statues and obelisks have been erected.

Designed to impress its visitors, the temple boasts twelve columns 21.17 meters (69 feet 5 inches) tall and two meters (six feet seven inches) in diameter, visually sitting atop a marble stairway that connects the base of the hill to the sanctuary.

An enormous fragment of entablature, weighing approximately one hundred tons and thirty four cubic meters in volume (the largest in Rome), belongs to the original temple, as do the statues of the Nile and the Tiber, moved by Michelangelo to the Capitoline Hill in front of the Senate building.