York and the five boroughs of the Danelaw
It has been said that the Vikings helped stimulate the growth of towns in England in three ways. Firstly, by the enlargement and fortification by the English of selected settlements in Wessex and Mercia, to defend against Viking attacks. Secondly, by the Vikings' own enlargement and fortification of towns within The Danelaw. Thirdly, by trade.
York (Jorvik) had a long history, from at least Roman times as a fortified settlement, regional capital, trading and religious centre, and this continued into the Viking era.
Five other settlements became particularly important after the establishment of The Danelaw when they became fortified 'boroughs' to help defend against English reconquest. These were Lincoln, Nottingham, Stamford, Leicester and (the only one renamed by the Vikings) Derby. These have become known collectively as `The Five Boroughs'.