London
Fortified city held by the Danes, with a castle and the fortified trading place of Southwark.
London is attested in the Heimskringla as a fortified city held by the Danes during the reign of King Ethelred. The Danes maintained a castle within the city, and on the opposite bank of the river they had built the trading place of Southwark (Sudvirke), which they fortified with ditches and a bulwark of stone, timber, and turf (Heimskringla, 11. Death Of King Svein Forked Beard).
The Heimskringla provides the sole attestation for London in this context, describing the city's defenses during the period of Danish occupation. The account is notably specific about the fortifications on both sides of the river — a castle within London proper and the separately fortified Southwark across the Thames, with its ditches and composite bulwark of stone, timber, and turf (Heimskringla, 11. Death Of King Svein Forked Beard). This dual-fortification detail conveys the strategic importance of controlling both banks of the river crossing.