Excerpts from Books and Wikipedia
In 885 Guthrum, the leader of the Danes, broke the agreement he had made with Alfred after the Battle of Edington, and invaded Kent with a large force. Alfred responded by capturing London, forcing the Danes to abandon their base in the south. A new agreement, known as the Treaty of Alfred and Guthrum, was reached that gave London to Wessex and redrew the frontier between Saxon England and 'Danelaw,' the area occupied by the Danes. A line now divided England into two halves. . . . South and west of the dividing line, Wessex was now recognized by all as the effective ruling power. Although the Danes would remain a constant threat until his death in 899, Alfred undertook important defensive measures to ensure Wessex's survival, including the building of a chain of fortified strongpoints with permanent garrisons to serve as refuge points for the population in times of war. Known as burhs ('boroughs'), these settlements were the origin of many of England's market towns. Alfred also had a number of large ships built to fight off coastal raids, for which he has sometimes been called the founder of the British navy." [1001 Days]