My guidebook entry for Longnai Hot Springs (瀧乃湯) (244 Guangming Rd, Xinbeitou, Taipei; admission NTD90/50; open: 06.30—21.00 daily) reads: Almost lost amid much taller and newer buildings, Longnai offers a thoroughly traditional bathing experience. There are two pools inside this slightly decrepit 70-year-old wooden bungalow, one for each gender; no swimsuits needed. The water is usually 38—42 degrees Celsius. From Longnai it’s less than 500m to Xinbeitou MRT Station.
All of the above is correct, but a few more words can be added: No towels are provided, and there's no WC inside the building - so go before you pay for admission.