Baden, anc. Aquae Helveticae, town (1990 pop. 15,718), Aargau canton, N Switzerland, on the Limmat River. A noted spa since ancient times, the town has hot sulfur springs. It is also a manufacturing center known for electrical-engineering works and other factories. The Swiss diet met at Baden from c.1425 to 1712. The Treaty of Baden (1714) complemented the Peace of Utrecht (see Utrecht, Peace of). Baden was the capital (1798–1803) of Baden canton under the Helvetic Republic. The castle of Stein, now in ruins, was once a Hapsburg residence.