Complex sentence: By the early 12th century, Coel had become associated with the city of Colchester in Essex based on a folk etymology that derived the city's name from "Coel" and "castrum", producing "fortress of Coel".
Split into simpler sentences: By the early 12th century, Coel had become associated with Colchester in Essex based on the idea that he was the source of the city's name. This folk etymology derived the name "Colchester" from "Coel" and "castrum", producing "fortress of Coel".