The development of Oxford Castle began in 1071, built for William the Conqueror. The Castle was strategically positioned near the river, on the western edge of the existing Saxon town defenses. In the 14th century the military value of the castle diminished and the site became used primarily for county administration and for detaining prisoners. Most of the castle was destroyed during the English Civil War and by the 18th century the remaining buildings were adapted for use as a prison. It remained an active prison until 1996, which is pretty shocking when you see the pictures of of the poor living conditions.
St George's Tower built in 1071. 
Views of the city from the top of the tower.
Inside the prison
Three prisoners would share this cell.
Below the prison is the crypt
For another view of the castle you can climb The Mound.
View of the castle from the mound.
St. George's Tower
