Zwingen Castle with bridge, Medieval fortress in Zwingen, Switzerland

Zwingen Castle is a medieval fortress in Switzerland that consists of two separate building groups arranged like islands in the river. Bridges over the Birse connect these parts, and a distinctive conical-roofed tower stands as the main landmark.

The castle was first recorded in 1312 and served the Ramstein family for many years under the rule of the Basel bishops. In the 15th century it returned to direct control by the Diocese of Basel.

The castle's name comes from the medieval town foundation, and the Oswald Chapel inside the walls was rebuilt between 1571 and 1575. The Ramsteins, ministerials of the Bishop of Basel, pushed the site toward a small town to rival Laufen; the empty space inside the wall ring marks where that town never grew.

The castle lies on islands in the Birs and is a five-minute walk from Zwingen station. The arms of the river between the islands are dry or filled in, but the complex is still encircled by the Birs. A covered wooden bridge links the eastern forecourt to the main castle; much of its original timber survives, and the dried riverbed beneath it is still visible.

The core castle once stood on its own small island, no longer visible as such; its round keep was the first major build, with later structures fused onto it. On the north side, several latrine bays used the Birs as natural flushing water.