The war was continued during the winter (1572-1573). In December the Spaniards marched to attack a fleet frozen up near Amsterdam. It was defended by a body of Dutch musketeers on skates, who, by the superior skill of their evolutions, drove the enemy back and killed great numbers of them. In consequence of this extraordinary combat, Alva ordered seven thousand pairs of skates, and directed his soldiers to be instructed in their use. Siege was then laid to Haarlem, which town, warned by the fate of Zutphen and Naarden, made a defence that astonished all Europe. A corps of three hundred respectable women, armed with musket, sword, and dagger, and led by Kenan Hasselaer, a widow lady of distinguished family, about forty-seven years of age, enrolled themselves among its defenders, and partook in some of the most fiercely contested actions. Battles took place upon Haarlem Lake, on which the Prince of Orange had more than a hundred sail of various kinds; till at length Bossu, whose vessels were larger, though less numerous, entirely defeated the Hollanders, and swept the lake in triumph (May 28, 1573). The siege had lasted seven months, and Frederick de Toledo, who had lost a great part of his army by hunger, cold, and pestilence, was inclined to abandon the enterprise; but he was kept to it by the threats of his father, and on the 12th of July Haarlem surrendered. Don Frederick had written a letter solemnly assuring the besieged that no punishment should be inflicted except on those who deserved it in the opinion of the citizens themselves, yet he was in possession of strict orders from his father to put to death the whole garrison, except the Germans, and also to execute a large number of the inhabitants. Between two and three thousand were slaughtered; three hundred were drowned in the lake tied by twos back to back.
The resistance of Haarlem and other places determined Alva to try what might be done by an affectation of clemency; and on the 26th of July he issued a proclamation in which Philip was compared to a hen gathering its chickens under the parental wing. But in the same breath his subjects were admonished not to excite his rage, cruelty, and fury, and they were threatened that if his gracious offers of mercy were neglected, his majesty would strip bare and utterly depopulate the land, and cause it to be again inhabited by strangers. So ludicrous a specimen of paternal love was not calculated to excite much confidence in the breasts of the Hollanders, and Alkmaar, the next town to which Don Frederick laid siege, though defended only by eight hundred soldiers and thirteen hundred citizens against sixteen thousand veterans, also resolved to hold out to the last extremity. Enraged at this contempt of what he called his clemency, at Haarlem, Alva resolved to make Alkmaar an example of his cruelty, and he wrote to Philip that everyone in it should be put to the sword. But the inhabitants made a heroic defence and repulsed the besiegers in many a bloody assault; till at length the superstitious Spaniards, believing that the place was defended by the devil, whom they thought that the Protestants worshipped, refused to mount to the attack, suffering themselves rather to be run through the body by their officers; and Don Frederick, finding from an intercepted letter that the Prince of Orange contemplated cutting the dikes and flooding the country, in order to prevent the place from being surrendered, raised the siege (October 8th) after it had lasted seven weeks.