In the Early Middle Ages war on land was characterised by the use of small war-bands of household troops often engaging in raids and low level warfare. The arrival of the Vikings brought a new scale of naval warfare, with rapid movement based around the Viking longship. The Birlinn, which developed form the longship, became a major factor in warfare in the Highlands and Islands. By the High Middle Ages, the kings of Scotland could command forces of tens of thousands of men for short periods as part of the "common army", mainly of poorly armoured spearmen and bowmen. After the "Davidian Revolution" of the twelfth century, which introduced elements of feudalism to Scotland, these forces were augmented by small numbers of mounted and heavily armoured knights. Feudalism also introduced castles