Kalbe Castle, Ruined medieval water castle in Kalbe (Milde), Germany

Kalbe Castle sits between two branches of the Milde River and displays remaining defensive walls, ditches, and four outer fortification areas that demonstrate its former strength. The overall layout appears as a system of multiple defensive rings that were meant to stop attackers at several points.

Built in the 10th century as a Saxon border fortress against the Wends, probably on the site of a Slavic walled fort in the Milde marshes. A nunnery founded under its protection was devastated by the Obotrites in 983 and lasted until 1121. Kalbe was one of four Altmark castle wards, with Tangermünde, Osterburg and Arneburg, guarding the Frankish colony and mission monasteries between the Elbe and Milde. In June 1240 Margrave John I of Brandenburg destroyed it during the Teltow and Magdeburg wars.

The name comes from the von Kalben family, documented in 1207 in connection with the castle, where its members served as ministeriales. From 1324 the von Alvensleben held the castle as a condominium of several family lines, jointly appointing a commander there until the 18th century.

The ruins stand on a marshy island of the Kalbesche Werder between two arms of the Milde, reachable on foot from Kalbe. Only the chapel, gate and dwelling house survive, so allow time to walk the former 360-metre castle area and trace the filled-in ditches and outer works. The site is a protected monument; the family von Alvensleben had it consolidated in 1903 to halt decay.

The castle was surrounded by three separate moats, an advanced water defense system that made use of the natural marshland. This triple protective layer makes it a rare example of how water was used in the fortress design of that era.