What monastery did William found on the battlefield to commemorate his victory?

The monastery William founded on the battlefield was Battle Abbey. William the Conqueror vowed to establish a monastery if he defeated Harold Godwinson, and the abbey was built at the site of the Norman victory in Sussex.

The foundation also served as an act of penance. Norman forces had killed thousands during the conquest, and Pope Alexander II ordered the Normans to do penance in 1070. William therefore linked his victory monument with religious atonement. The abbey church's high altar was traditionally placed where Harold fell.

Construction began during William's reign, but he died before the complex was complete. The church was finished around 1094 and consecrated under his son, William II, also known as William Rufus. Its first monks came from the Benedictine Abbey of Marmoutier in France.

Battle Abbey was dedicated to St Martin of Tours, along with the Holy Trinity and St Mary in the abbey's early dedication traditions. The surviving ruins and battlefield are now preserved as an English Heritage site near the modern town of Battle, East Sussex.