The genius of the Saxons appears to have been adapted to field warfare rather than to the construction or maintenance of strong military stations, for we find that when defeated they took refuge in natural fastnesses rather than in fortresses; the woods and marshes of Somerset, for example, protected Alfred from the pursuit by the Danes, and the last stand of these people against the Normans occurred in the fens and marshes about Ely. There is no account extant of a protracted resistance afforded by a Saxon fortress; that of London against the Danes may be attributed to the massive Roman walls there. It is unsatisfactory to be compelled to wander thus in the realms of conjecture, but it is probable that the kinds of defence varied in different places, since at Worcester Edward surrounded an ancient borough with a wall of stone. An oblique light, however, is thrown upon the subject by the presence in England of a few undoubted examples of fortifications erected at definite dates by another northern race, _i.e._ the Danes, who might be expected to fortify themselves somewhat similarly to the Saxons. [Illustration: THE DANISH BURH AT GANNOCK'S CASTLE, NEAR TEMPSFORD.] These marauders built burhs at Reading, Quatford on the Severn, and Benfleet, but by far the best now remaining are those at Willington and Tempsford on the river Ouse. At Willington the Danes proposed to establish their winter quarters in 921, and an extensive burh was thrown up for the purpose. It consisted of a large enclosure with inner and outer wards, high ramparts, and three wide ditches filled with water from the river. The most striking features, perhaps, were the two large harbours within the fortifications, designed to protect the Danish galleys. The Saxon king Edward, however, carried the place by assault and burnt the fleet. The discomfited Danes, much lessened in numbers, retreated up the river, and near the junction of the Ivel with the main stream threw up a smaller burh which now bears the name of Gannock's Castle, near Tempsford. The fort is an oblong area enclosed within a single fosse, and, what is very significant in face of later developments, a mound of earth stands within it near a corner, where the only entrance to the fort is found. Probably this mound was protected by palisades the same as the rampart, but Edward, flushed by his former success, stormed the burh and captured it with terrible loss to the routed garrison. [Illustration: PEVENSEY CASTLE.]