Marcus Porcius Cato (234 BC - 139 BC), the illustrious ancestor of Claudius's less than illustrious tutor, was a Roman general, statesman and magistrate, known for his upholding of traditional morality.
The Bibliotheca Apollinis was on the Palatine Hill in Rome, attached to Augustus's palace, and to the Temple of Apollo Palatinus. Parts of the temple survive, but the library does not.
Julia Augusti Filia (39 BC - 14 AD), the daughter of Augustus and second wife of Tiberius, was accused of adultery and licentiousness, and exiled, on Augustus's orders, to the island of Ventotene in 2 BC. Both Cassius Dio (in The Roman Histories) and Seneca (in On Benefits) refer to her conducting "revels" and "debaucheries" in the Forum, "and even on the Rostra," with a number of male partners. Seneca explicitly states that she prostituted herself.
The Rostra was a platform at the western end of the Forum, and was the point from which orators, including Emperors, addressed the masses. Although secular in nature, it had a near sacred significance to the Roman state. It was a place where no woman should ever have been seen, and on which no man should be seen without a toga. The equivalent would be a member of the British Royal Family fornicating in the middle of Trafalgar Square. This specific claim, however, may well have been apocryphal.
Elaine Fantham's book, Julia Augusti (Routledge 2006), gives a balanced historical account of her life).
The ballad itself is Grave's own, though composed in the irreverent style of the ballads that Roman soldiers are known to have sung at triumphal processions.
It refers to the Battle of the Teutoburg Forest in 9 AD, where General Publius Quinctilius Varus lost three entire legions (18,000 men plus auxiliaries) along with his own life.
Suetonius, in The Twelve Caesars, records that in the aftermath of the disaster Augustus 'was so disturbed that, for months at a time, he let his beard and hair grow, and would hit his head against the door, shouting: "Quintilius Varus, give me back my legions!"'