Subsequent Danish (Viking) invasion marked a period of destruction of many buildings in Anglo-Saxon England, including in 793 the raid on Lindisfarne. Buildings including cathedrals were rebuilt, and the threat of conflict had an inevitable influence on the architecture of the time. During and after the reign of Alfred the Great (871–899), Anglo-Saxon towns (burhs) were fortified. Contemporary defensive banks and ditches can still be seen today as a result of this. Oxford is an example of one of these fortified towns, where the eleventh century stone tower of St. Michael's church has prominent position beside the former site of the North gate. The building of church towers, replacing the basilican narthex or West porch, can be attributed to this late period of Anglo-Saxon architecture.