Burg Alsdorf, Medieval castle in Alsdorf, Germany.

Burg Alsdorf is a two-winged brick complex with a dark red coating, its right-angled wings two storeys high under slate roofs. The oldest part is the 23-metre round tower at the northeast corner, its walls 1.72 m thick at the base, topped by an octagonal slate-covered hood with a closed lantern. The south wing has two stepped gables and dates to 1724; the east wing, dated 1617 by wall anchors, has a small half-tower on its south end.

A Gottfried von Lovenberg is documented as Lord of Alsdorf in 1150; his line held the castle for seven generations until Hilger von Lovenberg died in 1404. His daughter Agnes married Arnold von Hoemen in 1417, and in 1478 the castle passed through Johanna von Hoemen to the von Harff family, who ruled for nearly two centuries. The von Blanckarts followed, and the town of Alsdorf became owner in 1935.

The name recalls the lordship of Alsdorf, held from 1150 by Gottfried von Lovenberg and later by the families von Hoemen, von Harff and von Blanckarts. Above the south door of the east wing, the alliance coat of arms of Alexander Adolf von Blanckart and Maria Florentina von Wachtendonk-Germenseel, with hammer and lily and the year 1723, marks their rebuilding. The park is open to the public free of charge.

The castle stands in the town centre of Alsdorf, surrounded by a small park that is free and open to the public. The main building now houses municipal facilities, so visits focus on the exterior and the grounds. Two surviving outer-castle buildings, the early 18th-century Remise and the gatehouse, stand near the main wings.

In the vaulted cellar of the early 18th-century Remise, twelve members of the Bockreiter gang waited in prison cells in 1775 for sentencing by the lord of the castle. The round tower, oldest part of the complex, is 23 metres high and its walls taper from 1.72 to 1.15 metres; it held the castle chapel until 1925.