Schauenburg Castle, Switzerland, Medieval castle ruin in Frenkendorf, Switzerland.

Schauenburg Castle is a medieval ruin situated on elevated terrain near Frenkendorf, with stone walls and foundation remains still visible. The fragments show the typical construction methods used for defensive structures during the Middle Ages.

Alt-Schauenburg was probably built around 1275 as the seat of a junior branch of the Schauenburg family. The 1356 Basel earthquake destroyed much of the castle, and it was abandoned shortly afterward because repairing it was judged not worthwhile next to the nearby Neu-Schauenburg. The name "Alt" refers to its ruined state, not its age. The Schauenburg line died out by 1385, and the site was forgotten until excavations in 1949–50 and 1976–77.

The name Alt-Schauenburg refers to the castle's ruined state after the 1356 Basel earthquake, not to its age: Neu-Schauenburg was built first, but Alt was abandoned and became the "old" one. The Schauenburg lords died out by 1385, and the site faded from memory until excavations in 1949–54 and 1976–77.

The ruin stands at 640 m on the Chleiflüeli rock head, about 500 m west of Schauenburg Bad; the approach is on foot and the south-west corner holds the entrance. Vertical rock faces close off the east, south and north sides, and the neck ditch and quarry hollows make the ground uneven. Excavation and conservation took place in 1949-54 and 1976-77; no work is ongoing.

The name Alt-Schauenburg does not mean the castle is older: Neu-Schauenburg was built first, but Alt was destroyed and abandoned after the 1356 Basel earthquake, so "old" refers to its ruined state. The Schauenburg family died out by 1385, and the site was only rediscovered by test digs in 1949–1954.