Oxford Castle is a large, partly ruined medieval castle on the western side of central Oxford in Oxfordshire, England. Most of the original moated, wooden motte and bailey castle was replaced in stone in the late 12th or early 13th century and the castle played an important role in the conflict of the Anarchy. In the 14th century the military value of the castle diminished and the site became used primarily for county administration and as a prison. The surviving rectangular St George's Tower is now believed to pre-date the remainder of the castle and be a watch tower associated with the original Saxon west gate of the city.

The Anarchy was a civil war in England and Normandy between 1138 and 1153, which resulted in a widespread breakdown in law and order. The conflict was a war of succession precipitated by the accidental death of William Adelin who drowned in the White Ship disaster of 1120. Henry sought to be succeeded by his daughter, known as Empress Matilda, but was only partially successful in convincing the nobility to support her. On Henry's death in 1135, his nephew Stephen of Blois seized the throne, with the help of Stephen's brother Henry of Blois, who was the bishop of Winchester. Stephen's early reign saw fierce fighting with disloyal English barons, rebellious Welsh leaders, and Scottish invaders. Following a major rebellion in the south-west of England, Matilda invaded in 1139 with the help of her half-brother Robert of Gloucester.