Valkenburg Castle is the partial ruin of a medieval castle on a sandstone ridge above the town centre. Construction began in the 11th century and the castle grew substantially over the following centuries before being deliberately destroyed in 1672. What remains — parts of the outer walls, the keep foundation, and various towers — is enough to convey the structure's original scale and gives good views over the Geul valley.
Access is via the cave system beneath the castle, which is part of the same ticket. The caves, carved from the local marl (a soft limestone), include a well-preserved section with medieval engravings. The combined castle ruin and cave tour runs daily and takes about 45 minutes. Wear a layer — the caves maintain a constant cool temperature regardless of outside weather.