Short answers, pulled from the story.
The Great Heathen Army landed in East Anglia in 865 with intentions to conquer four Anglo-Saxon kingdoms. This massive Danish force included leaders Halfdan Ragnarsson and Ivar the Boneless, sons of Ragnar Lodbrok.
King Alfred defeated Guthrum at the Battle of Edington in 878 before formalizing boundaries between 886 and 890 through the Treaty of Alfred and Guthrum. These agreements allowed Danes to keep their laws within their territory while maintaining loyalty to the English crown.
Leicester, Nottingham, Derby, Stamford, and Lincoln became the fortified towns that served as the strategic backbone of Danish rule. Four of these boroughs eventually became county towns for Leicestershire, Lincolnshire, Nottinghamshire, and Derbyshire.
Between 30% and 50% of the rural population held the legal status of sokeman under Viking law. This group occupied an intermediate position between free tenants and bond tenants, enjoying more autonomy than typical peasants.
Words like sky, window, and law entered common usage during this period along with third person pronouns they, them, and their. Place names across northern England still bear clear marks of Scandinavian settlement with endings such as -howe, -by, or -thorp.