Leil's son Rud Hud Hudibras reigned for thirty-nine years after him. Once the civil war was over, Hudibras restored peace once more to the people. It was he who built Kaerreint: that is, Canterbury. He also founded Kaerguenit, or Winchester, and the fortress of Paladur, which is now called Shaftesbury. [II.9]

CANTERBURY is another of the places said to have been founded by Rud Hud Hudibras. It appears in the Historia as Durobernia in the account of the Roman wars:

When [Cassivelaunus] heard of the arrival of Julius, he abandoned the siege [of Trinovantum] and hurried to meet the Emperor. As he marched into a valley near Durobernia, he saw there the Roman army busy pitching its camp and putting up its tents.... As they assaulted each other, Androgeus emerged from him grove and made a rear-attack upon Cassivelaunus' battle-line, on which the whole contest depended.... The forces of Cassivelaunus were scattered. He turned in flight and ran from the battle-field. [IV.9]

Later, Vortigern is at Canterbury when he hears of the landing of the Saxons:

At that moment Vortigern was at Durobernia, which is now called Canterbury, for it was his custom to visit that city frequently. [VI.11]

Tatlock remarks on Canterbury's lack of prominence in Geoffrey, noting how much more attention WINCHESTER receives, and speculates that contemporary tussles over episcopal supremacy might be at the root of this treatment (p. 36). The city's one appearance in Merlin's prophecies is in fact in an ecclesiastical context:

Religion shall be destroyed a second time and the sees of the primates will be moved to other places. London's high dignity shall adorn Durobernia; and the seventh pastor of York will be visited in the realm of Armorica. Menevia shall be dressed in the pall of the City of Legions, and a preacher from Ireland shall be struck dumb by a child still growing in the womb. [VII.3]

Apart from a brief reference to one of Arthur's many followers, Kynniarc, Duke of Durobernia [IX.12], that is all the mention of Canterbury in the Historia. Canterbury was in fact an enormously important see in Geoffrey's day; the website of Canterbury Cathedral is a good place to get a sense of its prominence. You might also enjoy The Canterbury Tour.