Castle Mill Stream is a backwater of the River Thames in the west of Oxford, England. Part of it runs parallel to the southern end of the Oxford Canal. It then flows under Hythe Bridge on Hythe Bridge Street, past the former Lion Brewery and under Oxpens Road to the Thames. The stream passed outside the original medieval city walls of Oxford, near Oxford Castle There was a mill here even before the castle existed
In the winter of 1142, there was a siege at the Castle Empress Matilda (aka Queen Maud, 1102–1167), the daughter and dispossessed heir of Henry I, was there during her power struggle with King Stephen (1096–1154). The queen escaped from the castle after her guards lowered her over the walls. She was dressed in a white dress that camouflaged her against the snow. Matilda crept through the enemy lines and across the Castle Mill Stream to escape to freedom.