He began by recognising the separate rights of the Mercians. He would not call himself King of Mercia. He placed his son-in-law Ethelred as Earl of Mercia, and because London was at that time considered a Mercian city, Ethelred took up his residence there as soon as the Danes had gone out. The condition of London was as desolate and as ruinous as that of the whole country. The walls were falling down: there was no trade: there were no ships in the river: no merchandise on the wharves: there were no people in the streets, save the Danish soldiers and the slaves who worked for them. Alfred restored the walls: rebuilt the gates: brought back trade and merchants: repaired the Bridge, and made London once more the most important city of his kingdom: its strongest defence: its most valuable possession. This was, in fact, the third foundation of London. If Alfred had failed to understand the importance of London—that great port, happily placed, not on the coast open to attack, but a long way up a tidal river, in the very heart of the country—a place easy of access from every part of the kingdom—a port convenient for every kind of trade, whether from the Baltic or the Mediterranean—the whole of the commercial history of England would have been changed, the island might have remained what it had been for centuries before the Roman Conquest, a place which exported iron, tin, skins, wool, and slaves, and imported for the most part weapons to kill each other with. Alfred gave us London. The lesson of ten years’ fighting taught Alfred what the Saxons had never before understood, the value of walled cities in the case of invasion. He saw—he was the first to perceive—how superior numbers may be rendered of no avail when they fling themselves against strong walls. The next Danish invaders found themselves stopped on their way up the Thames by a city fortified by a strong wall which the enemy could neither knock down nor climb over: and manned by citizens made doubly courageous by the safety and the strength of their ramparts. Six separate sieges were endured by London during the second invasion of the Danes: six separate times the enemy had to raise the siege and to go elsewhere, leaving London unconquered. Other walled towns were added—Winchester, York, Exeter, and Canterbury—but the first was London, whose fallen Roman wall, of which only the hard core of cement remained, Alfred rebuilt and faced again with stone.