Danish Vikings made several invasions in the 800s for which a danegeld tax on land was assessed on everyone every ten to twenty years. The amount was determined by the witan and was typically 2s. per hide of land. (A hide was probably the amount of land which could support a family or household for a year or as much land as could be tilled annually by a single plow.) It was stored in a strong box under the King's bed. King Alfred the Great, who had lived for awhile in Rome, unified the country to defeat the invaders. He established fortifications called "burhs", usually on hill tops or other strategic locations on the borders to control the main road and river routes into his realm. The burhs were seminal towns. They were typically walled enclosures with towers and an outer ditch and mound, instead of the hedge or fence enclosure of a tun. Inside were several wooden thatched huts and a couple of churches, which were lit by earthen oil lamps. The populace met at burh-gemots. The land area protected by each burh became known as a "shire", which means a share of a larger whole. The shire or local landowners were responsible for repairing the burh fortifications. There were about thirty shires. Alfred gathered together fighting men who were at his disposal, which included eorldormen with their hearthbands (retinues of men each of whom had chosen to swear to fight to the death for their eorldorman, and some of whom were of high rank), the King's thegns, shire thegns (local landholding farmers, who were required to bring fighting equipment such as swords, helmets, chainmail, and horses), and ordinary freemen, i.e. ceorls (who carried food, dug fortifications, and sometimes fought). Since the King was compelled to call out the whole population to arms, the distinction between the king's thegns from other landholders disappeared. Some great lords organized men under them, whom they provisioned. These vassals took a personal oath to their lord "on condition that he keep me as I am willing to deserve, and fulfill all that was agreed on when I became his man, and chose his will as mine." Alfred had a small navy of longships with 60 oars to fight the Viking longships. Alfred divided his army into two parts so that one half of the men were fighting while the other half was at home sowing and harvesting for those fighting. Thus, any small-scale independent farming was supplanted by the open-field system, cultivation of common land, more large private estates headed by a lord, and a more stratified society in which the king and important families more powerful and the peasants more curtailed. The witan became mere witnesses. Many free coerls of the older days became bonded. The village community tended to become a large private estate headed by a lord. But the lord does not have the power to encroach upon the rights of common that exist within the community.