The defence of the 9th-century kingdom of Wessex under King Alfred the Great 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 across the kingdom. They continued to form a basis of West Saxon rule during the period of the emergence of an English kingdom in the 10th century under Alfred's son, King Edward 'the Elder'. This title traces the development, construction and impact of these fortifications from the 9th to the 11th centuries AD.