The Tammisaari Sauna is actually a massive, five story complex filled with dozens of saunas of all different temperatures, plus cold and warm pools, showers and fountains and cleverly crafted artificial waterfals, plus a dark, cool, magical chamber known as The Void, the purpose of which is not generally known. Outsiders are rarely welcome, and the place is quasi-religious -- food and drink are not allowed, and men and women are kept segregated on separate levels - women on 1, 3, and half of 5, men on 2, 4, and half of 5.
There are healing and other restorative saunas here which can cure disease and other afflictions.
The Sauna is rumored to be usable for planar travel.
The Tammisaari Sauna is generally used by favor of the royal house - there is no fee, but permission is always needed.