The Emperor Claudius had a special relationship with the Caeilian Hill. Not only was the great Temple to his divinity built at its crest, but he built the great aqueduct which traversed part of it and brought bountiful fresh water to Rome.

Atop the Caelian hill, directly across a narrow valley from the Palatine Hill, the neighborhood was focused on its great Temple of Claudius [dedicated to the deified Emperor Claudius) and the temple courts that surrounded it.

Always a quiet, fashionable residential neighborhood, the Caeilian later became a center for the new Christian religion with its Church of St. John Lateran.

The Caelian is the most south-easterly of the Seven Hills of Rome and lies south of the Esquiline. It had two high points, referred to as the Larger Caelian (Caelius maior) to the west, and the Smaller Caelian, (Caelius minor) to the north. The Romans believed that Tullius Hostilius enlarged the city by adding the Caelian hill.

The Caelian originally was named Querquetulanus Mons. The earlier name was thought to be a reference to the oak trees with which the hill was covered. (Historians continue to dispute whether this represents a bona fide tradition or whether it simply was a later invention to explain the Porta Querquetulana.) The name Caelian derives from Caelius Vibenna, an early Etruscan warlord and adventurer who reputedly came to Rome in the time of Tarquinius Priscus and settled with his followers on the hill which later bore his name.

The Caelian was crossed by the Servian Wall, which traditionally was attributed to King Servius Tullius, but actually was built in 378 BC. It was densely populated during the days of the Republic. However, after a devastating fire in 27 A.D., the Caelian was occupied by the houses and extensive gardens of the wealthy. The chief buildings on the hill included the temple of Divus Claudius. The temple was almost entirely destroyed by Nero to build the nymphaeum which was part of his Domus Aurea. However, the temple of Divus Claudius was restored by Vespasian. Nero also constructed a new food market on the Caelian Hill. The Caelian served as the headquarters and barracks for several of the military units stationed in Rome, including the peregrini and the Frumentarii, the vigiles and the equites singulares (the mounted bodyguard of the emperor).

A major feature of the Caeilian, of course, was the massive aqueduct built by the Emperor Claudius, drawing abundant clear water from the hills far outside Rome itself.