Laarnes' castle gets its roots from a feodal watercastle from the 11th of the 12th century. From the orginal castle, the massive, square donjon-tower is a remainder. This tower is built in Balegemstone and has 2 turrets with stone spires. Via walls, this tower was linked to the round defensive towers, also carrying stone spires.
In the second half of the 17th century, the donjon itself got a pointed roof, and the walls were replaced by living rooms with large cross-barframe windows. At that moment, the crenels disappeared. The defensive system, with the drawbridge and herse disappaered. Instead, a fixed bridge with three arches came, along with a Baroque gatehouse.
The castle is however regarded as one of the better kept Middleage reinfoced castles in Flanders. Since 1943, it is protected. After a thourough renovation, since 1962 it serves as a museum, with a rich collection of wall carpets from the 16th century, furniture from the 17th and the 18th century, and silverwork from the 15th till the 18th century.