But while the assailants made desperate attacks upon the place [Dover Castle], the garrison were prepared for an obstinate defense, and offered a determined resistance at the points most open to attack. The conflict was maintained with fury on both sides for some hours of the day. But Eustace [Count of Boulogne] beginning to be doubtful of success, and being apprehensive of a sally by the besieged, which might force him to a more shameful retreat, gave the signal for retiring the ships. Upon this the garrison immediately opened the gates and falling on the rear guard with spirit, but in good order, killed a great many of them. The fugitives, panic struck by a report that the bishop of Bayeux had unexpectedly arrived with a strong force, threw themselves with their alarm among the crevices of the perpendicular cliffs, and so perished with more disgrace than if they had fallen by the sword. Many were the forms of death to which their defeat exposed them, many, throwing away their arms, were killed by falling on the sharp rocks; others, slipping down, destroyed themselves and their comrades by their own weapons; and many, mortally wounded, or bruised by their fall, rolled yet breathing into the sea; many more, escaping breathless with haste to the ships, were so eager to reach a place of safety that they crowded the vessels till they upset them and were drowned on the spot. The Norman cavalry took prisoners or slew as many as they could overtake. Eustace escaped by having the advantage of a fleet horse, his knowledge of the road, and finding a ship ready to put to sea. His nephew, a noble youth who bore arms for the first time, was taken prisoner. The English escaped through the by-roads, the garrison of the castle being too few in number to pursue a multitude who thus dispersed themselves.
Source: Ordericus Vitalis. The Ecclesiastical History of England and Normandy. Bk. IV, Ch. III, pp. 11-12.