Burg Grimmenstein, Medieval castle ruin in St. Margrethen, Switzerland.

Burg Grimmenstein is a hilltop castle ruin above St. Margrethen in the Swiss canton of St. Gallen. What remains is the corner of a residential tower, a cistern and wall fragments of other buildings; traces of the tower's high entrance are visible on the first floor, and the tower was more heavily fortified on the attack side.

The structure originated in the first half of the 13th century under the direction of St. Gallen abbot Berchtold von Falkenstein. After a change in its use, the castle was destroyed in 1416 and has remained a ruin since.

The ruins serve as a regional landmark showing how medieval fortifications were built and functioned in this area. Visitors can observe the remains and understand the role such structures played in local life.

The site sits on elevated terrain and can be reached on foot via a hiking path that offers varying difficulty depending on the route chosen. Sturdy footwear is recommended as the ground is uneven and can become slippery after rain.

In the 14th century, the fortress transitioned from an aristocratic residence to a hideout for bandits, marking an unusual turn in its history. This chapter ended with the structure's demolition in the early 15th century.