The most interesting feature, from a military point of view, of the contest between Wessex and the Danelaw, is the systematic defence of the Midland rivers by _burhs_ during the reign of Edward the Elder, either by himself or his sister Æthelflæd. Æthelflæd, who fixed her chief residence at Tamworth, on the edge of Staffordshire and Warwickshire, ruled over Mercia, and fortified her frontier between 909 and her death in 921. Her brother died in 925: his activity in constructing _burhs_ began about 913. Of the construction of these fortresses two phrases are used: their builders either “wrought” or “timbered” them. Both words probably mean the same thing: the town or village to be fortified was enclosed within the usual wooden stockade.[34] The identity of a few of the _burhs_ is uncertain; but the remainder may be classified as follows: (1) _Burhs_ wrought by Æthelflæd on the river banks of her frontier: these include Runcorn and possibly Warburton on the Mersey, Bridgnorth and possibly Shrewsbury on the Severn, Tamworth and Stafford on tributaries of the Trent, and Warwick on the Avon. (2) Frontier _burhs_ taken by Æthelflæd from the Danes were Derby and Leicester, both on tributaries of the Trent. (3) Eddisbury in Cheshire, an early hill fortress, was the site of one of Æthelflæd’s _burhs_: here the inference is that the existing hill fort was palisaded by her orders, and garrisoned as a camp of refuge. Of Edward’s _burhs_, Witham and Maldon on the Essex Blackwater, of which some probable traces remain, belong to class (1), as also does Thelwall, his fortified post on the Mersey. To class (2) belong Colchester, Huntingdon, and Tempsford, the first on the Colne, the two latter on the Great Ouse. None of Edward’s works bear any analogy to class (3), unless his last _burh_, Bakewell in Derbyshire, may be taken into account. But (4) Bakewell represents a push northward along a hostile border, and may be claimed, with Towcester, as belonging to a fourth class of _burh_, unconnected with a navigable river, but providing a constant menace to the enemy. (5) Towcester may, however, also be classed with Colchester as a Roman _burh_ with stone walls. A sixth class of _burh_ was riverine, like class (1), but with this difference, that it was double. There was one _burh_ on one side, the other on the opposite side of the river. The cases are Hertford on the Lea, Buckingham and Bedford on the Ouse, Stamford on the Welland, and Nottingham on the Trent. At Hertford and Buckingham both _burhs_ were the work of Edward. At Bedford, Stamford, and Nottingham, the northern _burh_ was in the hands of the enemy, and Edward took it by converting the southern suburb into a fortified and garrisoned post. His proceedings were exactly analogous to those of Charles the Bald in 862. He gained control of the navigable rivers by placing garrisons on both their banks; the natural places which he chose were the existing