Claudius, who had been raised to the position of Emperor, crossed over to the island. With him there came his chief of staff, who in his own language was called Lelius Hamo. It was this man who planned all the battles which had to be waged. Hamo landed in the city of Porchester and began to block up its gate with a wall, in order to stop the citizens coming out. He wanted to weaken them by hunger and in this way force them to surrender to him, or else he planned to kill them off mercilessly. [IV.12]

PORCHESTER is the scene of the emperor Claudius's invasion of Britain. It was indeed, as Tatlock points out (p. 49), the site of a Roman fort, part of the series of Saxon Shore Forts built in the 2nd and 3rd centuries. In Geoffrey's day, the fort was incorporated into new castle, built by Henry I around 1120. The Roman walls are still an important part of the site: visit this "unoffocial" Porchester Castle site for more information. You can also read about the castle at CastleXplorer, or on its English Heritage page.

While this was going on, Claudius had reassembled his troops. He attacked the aforementioned city of Porchester, which in those days was called Kaerperis. He soon broke down its walls and defeated its citizens. Arvirargus took refuge in Winchester and Claudius followed him there. [IV.14]

The British leader Arvirargus temporarily drives the Romans from the field: Geoffrey writes that Lelius Hamo takes refuge at a place on the coast now called Hampton-- this is Geoffrey's explanation for the origins of the name SOUTHAMPTON. Claudius then retakes the city, and Porchester's last appearance in the Historia is again in the context of conquest, when Constantine, now both king of Britain and emperor of Rome, puts down a revolt by Octavius, duke of the Gewissei:

The proconsul was killed and others with him, and Octavius seized the royal throne. This event was announced to Constantine and he sent Helen's uncle Trahern with three legions to restore the island to Roman sovereignty. Trahern landed on the coast near to a town called Kaerperis in the Welsh language. He attacked the town and captured it within two days. [V.8]