Heritage Open Day: This pinnacled gatehouse, elaborately decorated in East Anglian 'flushwork', is the sole survivor of the wealthy Benedictine abbey of St John. It was built around 1400 to strengthen the abbey's defenses following the Peasants' Revolt. This elaborate 15th-century gatehouse is all that remains standing of the Benedictine abbey of St John the Baptist that stood outside the walled town of Colchester. The extent of the abbey is still defined by the much-repaired precinct wall, and the gatehouse stands at the centre of the northern boundary. he abbey was founded in 1095 by Eudo Dapifer, William the Conqueror’s High Steward and Constable of Colchester Castle.