Geoffrey never actually mentions MONMOUTH, which we assume to have been his birthplace, directly in the Historia. He does however situate an important event in the area:

Aurelius Ambrosius.... marched his army into Kambria and made for the castle of Genoreu, for it was there that Vortigern had fled in his search for a safe refuge. This castle, which belonged to Erging country, was beside the River Wye, on a hill called Cloartius.... They lost no time, but moved into position with their siege-machines and did their utmost to break down the walls. When everything else had failed, they tried fire; and this, once it took hold, went on blazing until it burned up the tower and Vortigern with it. [VIII.2]

It would seem this is not the same tower that Vortigern built at DINAS EMRYS. Tatlock argues that Cloartius is a scribal error for Doartius, pointing to the prehistoric hillfort on Little Doward hill at Ganarew near Monmouth, and the traditional name King Arthur's Cave for the cave in Great Doward Hill (p. 72).