LINCOLN appears in the Historia, of course, in Geoffrey's dedication of the prophecies section to his patron, Alexander, bishop of Lincoln. Most of the famous Cathedral postdates the twelfth century, but the city and diocese were very important in Geoffrey's day. The city itself makes two appearances as the site of battles:

The Saxons went first to London and captured the city. Next they took York, Lincoln and Winchester, ravaging the neighbouring countryside and attacking the peasantry, just as wolves attack sheep which the shepherds have forsaken.[VI.16]

Arthur let a few days pass and then they marched to the town of Kaerluidoet, which was besieged by the pagans about whom I have already told you. This town is situated upon a hill between two rivers, in the province of Lindsey: it is also called by another name, Lincoln. As soon as they had arrived there with their entire force, keen as they were to fight with the Saxons, they inflicted unheard-of slaughter upon them; for on one day six thousand of the Saxons were killed, some being drowned in the rivers and the others being hit by weapons. As a result, the remainder were demoralized. The Saxons abandoned the siege and took to flight. [IX.3]

Tatlock points out that the second river in this decription would have to be the Foss Dyke, originally a Roman canal that had been restored to use in 1121, suggesting personal knowledge on Geoffrey's part (p. 26). It seems that Geoffrey is working from the battle-list in the Historia Brittonum here, where in c. 56, Arthur fights a battle super aliud flumen, quod dicitur Dubglas, et est in regione Linnuis: on a river called the Douglas, in the region of Lindsey.

There is also a reference to Lincoln in Merlin's Prophecies:

The Fox of Caerdubalum will wreak vengeance on the Lion and eat it up with its teeth. Then the Adder of Lincoln will coil round the Fox and announce its presence to the assembled Dragons with a terrifying hiss. [VII.4]