Stein Castle, Medieval fortress on Schlossberg hill in Baden, Switzerland.

Stein Castle is a ruined hilltop fortress on a rocky ridge of the Schlossberg at 445 m above sea level, overlooking Baden in the canton of Aargau, Switzerland. It is listed as a class A Swiss cultural property of national significance. The only fully preserved structure is the Sankt-Nikolaus chapel; the reconstructed watchtower, remains of the ring wall with a broken arch, and a long retaining wall are visible.

The castle was built in the medieval period as an administrative center for Habsburg territories and stayed under their control for generations. In 1415, Swiss forces took the fortress during their push into the Aargau region and destroyed it shortly after.

The ruin sits on a hill directly above Baden's old town and is visible from many streets below. Locals often walk up to enjoy the view over the rooftops and the Limmat river valley.

The site is reached on foot by climbing a steep staircase from Schlossbergplatz, passing through the stone gateway of the City Tower. The path demands some physical effort, so sturdy shoes are a good idea given the uneven steps and the incline.

The only fully preserved part of the fortress is the Sankt-Nikolaus chapel, which formed the forward anchor of the roughly 120-metre-long complex. From the chapel, remains of the old city wall run eastward down to the City Tower. The south-eastern shoulder bastion behind the chapel holds an underground casemate, through which the Nikolaustreppe climbs from the old town up to the fortress courtyard.