Short answers, pulled from the story.
Rouen derives from the Gallo-Romance form Rotomagus, itself from the Gaulish Ratumacos, a name associated with the Veliocassi tribe whose coins bore the word. The first element may connect to a root meaning "wheel" or "race", while the second element, magos, meant "field", "plain", or "market", so the combined name may have meant "hippodrome", "racecourse", or "wheel market".
Norse settlers who arrived following the creation of the Duchy of Normandy by Rollo in 911 introduced hundreds of place names built on Old Norse and Old English personal names and appellatives. These names were fixed before the eleventh century, while the Norse speakers were still a dominant presence before being absorbed into Old French society.
The suffix -tot is the most common suffix of Old Norse origin in Normandy, with more than 300 locations carrying it. It derives from the Old Norse topt, meaning "site of a house", and combines with personal names, tree names, and descriptive adjectives.
Norman -ville place names are distinctive because their first element is frequently an Old Scandinavian or Old Anglo-Scandinavian personal name, which does not appear in -ville names elsewhere in France. Examples include Amfreville from the Norse name Ásfríðr, Tocqueville from Tóki, and Tourville from Thori.
The suffix -acum derives from the Celtic -āko(n) and originally indicated the location of a god or a people, meaning roughly "place of" or "property". In Normandy and northern France it evolved into endings such as -ay, -ai, -ey, -é, and -y, found in places like Gournay, Bernay, Cernay, and Andilly.
Most settlers came from areas corresponding today to Denmark, with a smaller proportion from Norway. After the founding of the Duchy of Normandy, many also arrived from the Danelaw, and a further group, concentrated particularly in the Cotentin peninsula, came from Viking communities in the Scottish Islands and Ireland.