Sasso Corbaro, known as Unterwalden Castle after 1506 and Saint Barbara's Castle after 1818, is about 600 m (2,000 ft) south-east of town on a rocky hill. Unlike the other two castles Sasso Corbaro is not integrated into the city walls. The first part of the castle was the north-eastern tower which was built in 1478 to close a gap in the defenses of the city. In 1479 a small garrison was moved to the tower. During peacetime the tower was used as a prison, though at least one prisoner escaped in 1494.[16] The walls and south-west tower were added later. The castle was struck by lightning multiple times during the 16th and 17th centuries, and by 1900 was falling into ruins.