The other day, I walked out of the front door of The Oxford Hotel, in Denver, and strolled a few steps to The Oxford Salon and Club, which serves as the hotel’s spa and fitness center.

I didn’t think anything of heading outside, it was a nice day, and I got to walk Oxford Hotel Denver. Photo by Alison Stein Wellnerunder the hotel’s fabulous replica of its original wrought iron sign and towards the city’s beautiful train station.

The spa’s layout also struck me as pretty standard: small treatment rooms arrayed on either side of a narrow hallway. And the massage was top notch.

It was only later when I took a tour of the historic hotel—which dates back to 1891—that I learned about the salacious back story of my entirely innocent morning’s activities. It turns out there’s long been an indoor route to the spa, across the second floor of the hotel and down a discreet flight of stairs.

This was especially handy back in the day—when the spa was the hotel’s brothel. The entrance from inside the hotel was a way of maintaining client’s privacy.

The Oxford has been renovated several times throughout its long life, most recently to restore as much of the past as is practical and comfortable, while smartly updating the conveniences—so old-fashioned deep claw foot tubs in the guest room and metal keys for the door have stayed, while working fireplaces have given way to flat screen TVs and iPod docks. And the brothel has become a quite nice spa and fitness center.

The Oxford might be unusual today for its history, but back in the day, an on-or-near-premise brothel wouldn’t have been at all unusual—this was practically a standard fixture of 19th century hotels.  In New York City in 1830, for instance, 80% of houses of prostitution on Manhattan’s West side were within two and half blocks of a large hotel. This fact I found in Hotel: An American History, by A.K. Sandoval-Strauz.

I also learned that one of the earliest guidebooks was an 1839 booklet called Prostitution Exposed, which purported to be:

a MORAL REFORM DIRECTORY, Laying Bare the Lives, Histories, Residences, Seductions, &c. of the most celebrated COURTEZANS AND LADIES OF PLEASURE of the city of New-York, Together with a Description of the Crime and Its Effects, as also of the Houses of Prostitution and the Keepers, HOUSES OF ASSIGNATION, Their Charges and Conveniences, and other particulars interesting to the public.

But was, in fact, a guidebook listing detailed street addresses for said houses of assignation. The best part was the pseudonym the author selected: Butt Ender.

Yes. The book was known as Butt Ender’s Prostitution Exposed.

A moral reform directory, indeed.