The Anglo-Saxon kingdoms faced a major threat from Viking invasions and as a result many towns were fortified against attack. These fortified towns, known as burhs, were not very sophisticated but they were effective. The first step in constructing a burh was to dig a very deep trench, and then to build a wooden or stone wall around the town. Inside the walls the burhs were not very different to any other town except for the large gates on either end of the town that controlled who came in and out.