The Sultaniye springs sit on the western shore of Köyceğiz Lake, a short boat ride or drive north of the mud baths. The waters here are hot — around 39°C — and rich in sulphur, chlorine, and minerals. The site has been used since Roman times; remnants of older bathhouses are still visible in the masonry around the modern pool.
The main bath is housed in a domed Ottoman-era building with a central pool. There are also outdoor mineral pools and a separate area for mud applications. It's quieter and less circus-like than the busier mud baths nearer the river, and worth the extra distance if you want a proper soak.