During ground penetration radar measurements, experts from the Westphalia-Lippe Regional Association (LWL) discovered, among other things, the remains of a mighty tower, the keep. This round tower was probably free in the courtyard. With a height of 25 meters and an outer diameter of around 14 meters, it was one of the larger examples of such towers that were part of the core of every castle from the 12th to the 15th century.
Horstmar Castle was the seat of the noblemen of Horstmar, first documented in 1092. In 1269, the Horstmar estate including the castle was sold to the Münster monastery. The castle was now the center of the episcopal office and the judicial district of Horstmar. Expanded into a state fortress against the County of Steinfurt since the 15th century, it remained a favorite seat of the bishops of Münster until the 16th century. According to written records, the castle at that time consisted of a high tower, a chancellery, a residential wing with rooms for the sovereign and functionaries, a prison tower and various outbuildings, including a construction and brewery, a bakery and stables. During the Thirty Years' War the castle was destroyed in 1635 on the orders of the Hessian lieutenant Rabenhaupt. The last aboveground remains were probably removed in the 18th century.
(Source: lwl.org).