William’s operations in 1068 and 1069 were of great military importance. In 1068 he quelled the resistance of Exeter. The city was still surrounded by its Roman walls, to which the inhabitants now added new battlements and towers. They manned the rampart walks and the projections of the wall,[44] which for eighteen days William endeavoured to undermine. When at last the keys of the city were surrendered to him, his first work was to choose within the walls a place where a castle might be raised; and, on departing, he left, as at Winchester, a constable in charge of the castle, the king’s lieutenant charged with the task of keeping the _burh_ under. From Exeter a rebellion in the north called William to York. The insurgents, an irregular band of freebooters, had thrown up defences in remote places in woods and by the mouths of rivers; some were harboured in the larger towns, which they kept in a state of fortification. As William travelled northwards, he founded castles at Warwick and Nottingham. He constructed a fortress in the city of York, and on his way home founded castles at Lincoln, Huntingdon, and Cambridge. No sooner had he left York than the rebels again began to stir; a movement was made on behalf of the ætheling Edgar, and Danish aid was called in. William Malet, the governor of York castle, was hard pressed by the enemy. The Conqueror came to his relief, and, as a result of this visit, founded a second castle in York. Both castles, however, were of little use when the Danes came. The garrison of one or both rashly advanced to fight the invaders within the city itself, and were massacred. It is a significant fact that the castles were left open and deserted; neither the men of York nor the Danes had any use for them. When William came north again on his campaign of vengeance he repaired both the castles. Shortly after, on his expedition to Wales, he founded castles at Chester and Shrewsbury.[45] [Illustration: Lincoln; Plan]