Warfare was endemic in early Anglo-Saxon England, and major conflicts still occurred approximately every generation in the later period. Groups of well-armed noblemen and their households formed the heart of these armies, supported by larger numbers of temporary troops levied from across the kingdom, called the fyrd. By the 9th century, armies of 20,000 men could be called up for campaigns, with another 28,000 men available to guard urban defences. The most common weapon was the spear, with swords used by the wealthier nobles; cavalry was probably less common than in wider Europe, but some Anglo-Saxons did fight from horseback. The Viking attacks on England in the 9th century led to developments in tactics, including the use of shield walls in battle, and the Scandinavian seizure of power in the 11th century introduced housecarls, a form of elite household soldier who protected the king.