It would seem that the system by which Alfred’s “burhs” were maintained was not unlike that which Henry the Fowler employed in Germany a generation later. To each stronghold there was allotted, as it would appear, a certain number of “hides” of land in the surrounding region. All the thegns dwelling on these hides were responsible for the defence of the burh. Probably they were bound to build a house within it, and either to dwell there in person, or to place therein a substitute equally competent with themselves for military purposes. It would seem that the “cnihten-guilds” of London and several other places were the original associations of these military settlers whom Alfred and his immediate successors placed in their burhs. Of the local distribution of the fortresses we have a precious relic in the “Burgal Hidage,” a document belonging to the very early years of the tenth century, which gives a complete list of all the land dependent on the burhs of Wessex, and certain materials for the regions north of Thames also, where Edward the Elder was beginning to encroach on the Danelagh by means of his new foundations. That the system started with Alfred rather than his son seems to follow from two passages in the Anglo-Saxon Chronicle, where, under the year 894, we hear of “the king’s thegns who were at home in the fortresses,” and again of the fyrd being “half in the field and half at home, beside those men that held the burhs.” One of Alfred’s devices of fortification deserves a special mention, as being new on this side of the Channel, though some partial precedents for it can be found in the wars of the Franks. In 896 the main body of the Viking invaders had concentrated at the Thames mouth, and then pushed up the river Lea to a spot fifteen miles from London, dragging their fleet with them. Noting the narrowness of the river, Alfred built two formidable burhs, one on each side of the Lea, just below the Danish camp, and then obstructed the stream—probably by palisades and floating booms—between the two forts. The hostile fleet was so securely “bottled up” that the Vikings had to abandon it when they moved off on land, and the Londoners were able to bring back the whole of the galleys to their city when the enemy was gone.