A huge army was assembled and ordered to march to the city of Bangor and destroy Abbot Dinoot and the other churchmen who had scorned Augustine. They accepted Ethelbert's orders, collected an enormous army together and set out for the land of the Britons. They came to Chester, where Brochmail, who was in command of that city, awaited their coming. A great number of monks and hermits from all the different territories of the Britons, and especially from the city of Bangor, had sought refuge in Chester, so they could pray there for the people's safety. Armies were drawn up on both sides, and Ethelfrid, King of the Northumbrians, joined battle with Brochmail. Brochmail stood firm against him, although his force was smaller. In the end, however, Brochmail abandoned the city and fled, but only after inflicting enormous losses on the enemy. When Ethelfrid occupied the city and discovered the reason why these monks whom I have mentioned had come there, he immediately let his soldiery loose against them. That same day twelve hundred monks won the crown of martyrdom and assured themselves of a seat in heaven. [XI.13]

All the princes of the Britons then assembled in the city of Chester and agreed unanimously that they should make Cadvan their King and that under his command they should cross the Humber in pursuit of Ethelfrid. Once Cadvan had been crowned King of the realm, these princes gathered from all sides and crossed the Humber. [XII.1]

CHESTER is probably to be identified as the site of the massacre of the monks of BANGOR, though the name used is Legecestria. Tatlock points out that there is confusion over the forms Caicestria and Legecestria. The former, which suggests the more common Cestria, for Chester, occurs only as a title (Cursalem of Caistor attends Arthur's plenary court, IX.12, and is present at the Battle of Saussy, X.6). By Geoffrey's use of the latter, Tatlock argues we should understand LEICESTER-- both cities were called Legecestria (pp. 24-25). While Tatlock assigns all the instances of Legecestria in the Historia to Leicester, I have here followed Lewis Thorpe in placing the massacre of the Bangor monks (in a place which, Tatlock notes, Bede called Legacaestir, p. 26, meaning Chester) in Chester. Tatlock himself remarks on Geoffrey's apparent ignorance of geography in the unlikelihood of the monks' travelling all the way to Leicester from Bangor (whether Bangor on the coast, or BANGOR ISCOED), and this is not one of the instances where Geoffrey stipulates that "Legecestria" is on the River Soar-- as Leicester indeed is.

Chester was an important frontier town in Geoffrey's day. There is an illustrated overview of the city's history on the Cheshire Now website.