D'Oyly positioned his castle to the west side of the town, using the natural protection of a stream off the River Thames on the far side of the castle, now called Castle Mill Stream, and diverting the stream to produce a moat.[9] There has been debate as to whether there was an earlier English fortification on the site, but whilst there is archaeological evidence of earlier Anglo-Saxon habitation there is no conclusive evidence of fortification.[9] Oxford Castle was an "urban castle", overlying a portion of the Saxon town wall, but it remains uncertain whether local buildings were demolished to make room for it.