The Greeks and the Romans played up this concept of water water springs. There were medicinal benefits tied to usage, and if you were of a better status in life.....you tended to use the springs more than the regular guy on the street.
When the Romans came north into Germany and did their element of conquering around 12 BC. Control by the Romans over the Wiesbaden area? There's some discussion over the date....but most side that around 120 AD....the Romans had some type of control over the local region. What most will say is that the warm springs of the local area drew the Romans, and it was the magnet for regional development of the baths.
All of this came to some evolution of sorts when the regional tribes did a partnership with the Romans....to help them eliminate any threat...coming around the year 370. For around 120 years....this partnership survived, and was eventually defeated by the Franks.
You can eliminate Roman control over the spa business in 496, and with the Franks....came the curious episode of making Wiesbaden a capital city of the Franks.
For the next thousand years....the Catholic church plays some part in role of Wiesbaden, and it's spa business.
Around the early 1800s....there's stability....and medicinal advice being given by 'experts' over the benefits of warm water bathing. There's minor tourist industry developing.
By the 1850s....between the spa operations, the casino, and the grand hotels....Wiesbaden became a thriving spa resort.
If you were a major hotel in town.....you had bathing rooms for the guests to reserve. The average room was around 15 feet by 30 feet, with a fairly good sized tub in the back of the room. A window was typically in each room, with nicer grades of tile on the floor. The local geothermal waters were pumped in, and a guy would sit for approximately forty-five minutes (one hour periods were the norm, but the bath lady always had to clean it and prepare for the next guest). So you paid for the water treatment, and did that several times in a week as your "kur".
You would have the bathing room set for you as you enter....the bathing lady would leave. You'd undress and lay in a tub with greatly heated water. Maybe later....you'd swim in a local pool, and talk London or Paris gossip with the guys in the sauna. That night, you'd sit around the Casino....gamble a little, and talk politics. Maybe on a warm afternoon.....you'd have some ice cream or cold beer to replenish your liquids. You relaxed.....you talked business.....you took a holiday from life.
The Hotel Zais and the Hotel of the Four Seasons....ran combined bath house deal with forty-four bathing rooms (they were opposite of the casino).
The Eagle Hotel had around seventy-five bathing rooms.
The Nassauer Hof had thirty-six bathing rooms.
The Rose Hotel ran fifty-one bathing rooms.
After those five hotels.....there were structures simply for bathing and had nothing to do with the resort hotel operations. All total? Around twenty-three business fronts operating as spa centers, with around five hundred bathing rooms.
Between the upper-class hotels, the four-star establishments, and the slightly lesser hotels.....the entire town was bustling on bathing water "sales".
What came out of this period was periodic journals or magazines....in Germany, France, Italy, Switzerland, Austria, the US, England, and Russia.....about the robust health aspects of laying in warming waters, with the right content of natural elements. Doctor "so-v-so" would talk up the magical numbers of potash, sulphur, and Silicic Acid. All of these cured body ailments, muscle pains, and restored youth.
Added to the list of bathing cures......Wiesbaden had an ample number of doctors in the area which treated your special problems (cysts, blood circulation in your legs, back pain, and graying hair). You pick it.....they were willing to help fix your problem....for a price.
All of this was a resort magnet, and naturally brought the rich and wealthy from across Europe to Wiesbaden....almost yearly. A guy would pack up his wife in London....take a ship over, and travel by boat or train down to Frankfurt.....then over to Wiesbaden by the Taunus Railway. If you could afford the upper-scale hotels, great. If you needed less.....there were ample accommodations in town to handle just about every category of spender.
In a sense....for roughly seventy-odd years.....it was a status symbol to say you were a banker out of London, and spent three weeks in Wiesbaden every summer for your health issues and stress. Maybe between the walks in the city park....the fine food and wine.....the bathing waters....and just the quiet nature of the city.....you got better. Or at least you believed that.
In the summer of 1914.....that resort spa status ended. Except for Germans.....for the next forty-odd years....no one came back. By the 1960s.....Wiesbaden had some resort tourism going on, but the health spa status? It's mostly gone.
Without WW I/WW II.....Wiesbaden becomes an interesting topic of discussion. The resort status would have grown. The population of the city would likely be double what it is today (281,000). And the city would be consumed over health, bathing, fine foods, and eliminating stress in every conceivable way.