The second expedient which Alfred took in hand was the systematic construction of fortifications. Not only were the towns encouraged to surround themselves with strong ditches and palisades, but “burhs”—moated mounds girt with concentric rings of ditch and stockade—were erected at strategical points. London, recovered from the East Anglian Danes in 886, was made far stronger than it had ever been before by the patching up of its ancient Roman walls. It was filled with a new colony of warlike settlers, and became an outpost of Wessex to the north of the Thames. The consequences of the fact that the larger English towns were no longer open but well fortified are clearly seen in Alfred’s later wars. The Danes cannot capture important places at the first rush, as they had done with York, Winchester, and London thirty years before. They have to lay siege to them in full form, and always before the siege is many days old the indefatigable king appears with an army of relief. The invaders had then either to fight, to take to their ships, or to stockade themselves in their entrenchments and suffer a leaguer themselves. Generally they chose the second alternative, as at Rochester in 886, when they abandoned their horses, their stores, and all their heavy plunder, and sailed off the moment that the army of succour came in sight. The same scene occurred at Exeter in 894. The importance of fortified places in keeping the Danes employed till the fyrd could assemble can hardly be exaggerated. The only stronghold which did not serve its purpose was a certain “work only half constructed in which there were some few countryfolk” near Appledore in Kent. This fell before an attack of the “Great Army” in 893.