(Oudevaartsgat 8) is a canal-shaped L-shaped castle with a round corner tower and two smaller square towers. It was founded in 1288 by Floris V as a compulsory castle. Originally it had the shape of a square castle with round corner towers and a small square tower in the middle of each side. After the dismantling in 1578, the remnants came to the city in 1606. From 1661 to 1734 the great hall served as 'Oosterkerk'.
The ruins were demolished in 1758 and 1848, leaving only the southern quarter. That came to the State in 1887. A drastic recovery and conversion to canton law followed in the period 1890-'97 to plans of J.A. van Lokhorst with advice from P.J.H. Cuypers. The name Radboud castle was derived from the alleged 8th century founder of Medemblik. During the restoration, the outside cladding was renewed, the southern round tower rebuilt and the whole provided with stepped gables and battlements. After demolishing the steam cannery factory at the castle (1906, fire 1931) and surrounding buildings in 1936, the castle canals were excavated again in 1937 and the contours of the original castle were indicated. In a further restoration in 1967 (H. de Lussanet de Sablonière), late-19th century interventions were corrected on the basis of newer insights. Inside there are two late gothic fireplaces from elsewhere on carved cheek pieces (circa 1450) and an early Renaissance tour portal (second half of the 16th century).
