The giant stone tower perched on a hill to the east of the city centre, is the ruined remains of Šalek Castle, the oldest castle in the Šaleška valley. Abandoned in the late 18th century, the tower is nevertheless one of the most impressive sights in Velenje, boasting a unique triangular layout that is the only of its kind in Slovenia. The lookout also affords spectacular views of the city, and is free to enter - although you'll need to enquire at the tourist info centre to get the key to the outer gate.