Many of the _burhs_ wrought or taken during the Danish war became, after the Norman conquest, sites of castles; and the presence of a Norman castle at such places has led to the still popular inference that the castle simply usurped the earthworks of the earlier stronghold, and that therefore the _burh_ was equivalent to the later castle.[35] It is not surprising that all the five places where we hear of a _burh_ on either side of the river should have been chosen for the foundation of later castles. But the castle earthworks of Hertford and Bedford, the castles of which there is record at Buckingham and Stamford, were private strongholds which formed part of the defences of one of the _burhs_, but were not identical with either. The Conqueror’s castle of Nottingham, greatly transformed in its present state, looked down from its sandstone cliff upon the northern _burh_ where Edward welded together Englishman and Dane in one common work of defence and bond of citizenship.[36] But even were it not self-evident that the _burh_ is identical with the _burgus_ or _burgum_ of Domesday and the “borough” whose organisation plays so large a part in English history, there is one fact which makes its identification with the castle impossible. At Hertford, Buckingham, Bedford, Stamford, Nottingham, there were two _burhs_, but there never has been more than one castle. When the Conqueror wrought a castle on either side of the river at York, he did not repeat Edward the Elder’s tactics literally: he applied them to a form of fortification of which Edward knew nothing. If the test by which the Norman castle is identified with the Saxon _burh_ fails in these instances, we are obviously forbidden to make the identification in the cases of single _burhs_ like Warwick or Tamworth. The great earthen mount and curtain wall which stand in the south-west corner of Tamworth, beside the Tame, are not the remains of Æthelflæd’s _burh_, although it is not improbable that they were raised on the site of her dwelling-house. Her _burh_ is the town of Tamworth itself; and although her wall of palisades is gone, there are still traces of the ditch with which, in the eighth century, Offa had ringed the _burh_ about. [Illustration: Earthwork at Tempsford.]