Howbury Ringwork, also known as Renhold Ring or Addingreves Castle, is a medieval fortification in the village of Renhold, Bedfordshire.
The ringwork is 40 m in diameter, protected by an 8 m wide, 3 m high earth bank, and a 24 m wide wet ditch, probably originally several metres deep. It was probably constructed in the 11th century after the Norman conquest of 1066, although no records survive to confirm this. A later medieval droveway for moving cattle cut across the site and a modern road has damaged its northern edge.
The ringwork is protected under UK law as a Scheduled Monument.