Diepenstein Castle, Castle in Londerzeel, Belgium.

Diepenstein Castle is a red-brown brick complex of two joined wings beside a pond in Steenhuffel, Londerzeel. The tall left wing has a steep dark grey hipped roof with small dormer windows; the longer two-storey right wing has rows of rectangular windows with dark frames and white mullions, a steep roof with dormers and two tall brick chimneys, and a round tower with a conical dark roof at its right end. A dark horse statue stands on a stone terrace above a brick retaining wall along the water.

A wooden watchtower was built on the site in 1147 by Wouter van Craynhem as part of the defence line of the House of Brabant. A stone keep replaced it in 1290, the water castle was finished in 1357, and a gatehouse with drawbridge followed in 1460. The castle wall came down in 1517 for a pleasure castle; the surviving wing is 17th-century, and the gatehouse, coach house and stables were demolished in 1825.

The name Diepensteyn refers to the lordship centred here, which became a county under the Counts of Maldeghem in 1685. The castle is a protected monument, and since 1989 it has belonged to NV Diepensteyn, linked to the Palm brewery; the company restored it from 1993 to 1995 and added further renovations from 2018.

The castle stands in Steenhuffel, part of Londerzeel, at the end of a lane off the Diepensteynstraat area, with its brick wings rising above a pond and a brick retaining wall. It is private property of NV Diepensteyn, so the exterior is seen from the public road and the water's edge; the grounds and garden are not open to walkers.

The brick castle visitors see today is only the 17th-century remnant: in 1825 the gatehouse, coach house and stables were demolished and the surrounding ponds and moats filled in. A pond was dug again around the castle during the 1993–1995 restoration.