 Having seized York in that year, William the Conqueror built a castle on the south side of the city close to the River Ouse.Then, as a response to a rebellion the following year, a second castle was built on the opposite side of the river.There is no clear evidence which of these castles was built first, but it is generally thought to be the one which stood on the site of the later York Castle on the east side of the river, followed by the Old Baile on the west side.