In 865, a large Viking force known in later sources as the “Great Heathen Army” arrived in East Anglia and began sustained campaigns across several Anglo-Saxon kingdoms. Over the following decade, Viking armies broke the power of Northumbria, East Anglia, and much of Mercia. By the late 870s, Wessex was the main remaining kingdom able to resist, setting the stage for Alfred’s defensive reforms.
In mid-January 878, the Viking leader Guthrum launched a surprise attack and took Chippenham, forcing King Alfred into a crisis. Alfred escaped and regrouped from the Somerset marshes, using fortified positions and local support to rebuild his fighting force. This period helped drive a shift from short-term campaigning to longer-term planning for defense.
In May 878, Alfred met Guthrum’s army at the Battle of Edington in Wiltshire and won a decisive victory. The result forced the Vikings to negotiate rather than continue their attempt to dominate Wessex. This turning point made it possible for Alfred to invest in a kingdom-wide defensive network instead of relying only on emergency field armies.
After the fighting in 878, Guthrum accepted baptism during negotiations and agreed to leave Wessex, moving first into Mercia and then to East Anglia. Although later tradition calls this the “Treaty of Wedmore,” modern historians note that the surviving formal text is a different document made later. The withdrawal reduced immediate pressure on Wessex and gave Alfred time to rebuild defenses and administration.
In the years after 878, Alfred developed a system of fortified places called burhs (fortified towns or forts). Burhs provided refuges for nearby populations and secure points for garrisons, supplies, and communication. They also encouraged trade and craft activity, because protected places were safer for markets and workshops.
In 886, English control in London was restored, and sources say Alfred “restored” the city and made it habitable again within the old Roman walls. London (often described as Lundenburh) became a key fortified center and political symbol. Control was then associated with Mercian leadership, helping keep cooperation between Wessex and allied English regions.
By 886, Alfred and Guthrum reached a further peace agreement now known as the Treaty of Alfred and Guthrum. It set boundaries between their territories and included rules for trade and legal disputes, showing an effort to manage coexistence as well as conflict. This kind of settlement supported urban life by making cross-border exchange more predictable.
In 892–893, a major Viking force arrived again, with one group establishing itself at Appledore in Kent and another at Milton Regis. Sources note that families came with the army, suggesting an attempt at longer-term settlement rather than a quick raid. Alfred’s ability to contain these forces was tied to his broader defensive preparations, including fortified places and coordinated regional response.
Between 893 and 896, fighting spread across southern and western England, with Vikings also threatening fortified places such as Exeter. A key point of the burh system was that garrisons could hold attackers in place while field armies assembled and moved to relieve pressure. The result was not a single decisive battle, but a defensive framework that reduced the Vikings’ ability to overrun Wessex quickly.
In the early 10th century, Alfred’s son Edward the Elder and his sister Æthelflæd (Lady of the Mercians) used burh building and repair as a forward strategy, not just a defensive one. Fortified sites helped secure newly taken territory and limit Viking movement through river valleys and road corridors. This approach connected military gains to lasting settlement patterns, supporting more permanent urban and administrative growth.
The Burghal Hidage is a key document listing burhs and assigning each one a number of hides (a land-assessment unit) used to calculate manpower obligations for defense and wall maintenance. Many scholars place the document’s compilation in the early 10th century, likely after 914, while still seeing the overall system as rooted in Alfred’s reforms. It provides rare evidence that fortifications, taxation, and military service were being planned as a single integrated system.
By the end of the 910s, West Saxon rule had expanded across much of southern England, with fortified towns helping hold territory and support administration and supply. In 927, King Æthelstan (Alfred’s grandson) took control of Northumbria and could claim a wider kingship over the English. The burh system did not “finish” unification by itself, but it helped create the defended, governed spaces that made larger-scale rule more workable.
Fortified burhs, urbanism, and Alfred's defensive network (c. 870–950)