[Belinus] summoned workmen from all over the island and ordered them to construct a road of stones and mortar which should bisect the island longitudinally from the Cornish sea to the shore of Caithness and should lead in a straight line to each of the cities on the route. He then ordered a second road to be built, running west to east across the kingdom from the town of St Davids on the Demetian Sea over to Southampton and again leading directly to the cities in between. He built two more roads in a diagonal pattern across the island, to lead to the cities for which no provision had been made. Then he consecrated these highways in all honour and dignity, proclaiming it to be an integral part of his code of laws that punishment should be meted out to any person who committed an act of violence upon them. If anyone wishes to know the full details of the highway code established by Belinus, he must read the Molmutine Laws which the historian Gildas translated from Welsh into Latin, and which King Alfred later rewrote in the English language. [III.5]

SOUTHAMPTON appears first in the Historia as part of Belinus's network of roads-- an early indication of the importance of the location in both the Historia and in Geoffrey's own day. The etymology offered by Geoffrey (who usually renders the name as Portus Hamonis) attributes the name to Lelius Hamo, who he says was Claudius's commander at the time of the Roman invasion:

Arvirargus thought that Claudius had gone with Hamo and so he hurried off after him, pursuing him relentlessly from place to place until he came to grips with him and his men on the sea-coast at a place now called Hampton, for Hamo's own name. There was a port at this place, convenient for those who wished to land; and there were merchant ships drawn up on the shore there. Hamo did his utmost to climb on board these ships, but Arvirargus intervened and killed Hamo out of hand. From that day to this the haven has been called Southampton. [IV.13]

Tatlock points out (p. 48) that this figure is unknown, though we do in fact know a fair bit about this particular period in history. The "portus" part of the name is a fair reflection of Southampton's major significance, in the Historia and in Geoffrey's day both: it features as a major port for Conanus, for Hoel of Brittany when he comes to King Arthur's aid, and for Brian, the nephew of Cadwallo, near the end of the Historia:

Conanus immediately assembled all the young men of the realm and came to Southampton, where Maximianus had pitched his tents. [V.10]

... as soon as he heard of the terrifying way in which his uncle was being treated, Hoel ordered his fleet to be made ready. Fifteen thousand armed warriors were assembled and at the next fair wind Hoel landed at Southampton. Arthur received him with all the honour due to him, and each man embraced the other repeatedly. [IX.2]

Brian landed at Southampton and disguised himself as a beggar by putting on the clothes of some poor wretch. [XII.7]

The site Portcities includes an extensive section on the history of Southampton.