Book excerpt: The defence of the 9th-century kingdom of Wessex under King Alfred against the 'Great Viking Army' is one of the major military achievements of Early Medieval history. While the guerrilla warfare in the Somerset marshes and the battle of Edington are characteristic of Alfred's military abilities, his definitive physical achievement was a series of some 30 well-structured fortifications (known as burhs) across the kingdom. Many of these fortifications survive to this day and some were even reinforced to stand up to German tanks in the expected invasion of 1940. This title describes their beginnings in the turbulent early years of Alfred's reign as well as their subsequent development and use.