When Amos Richards Eno (1810-1898) planned the Fifth Avenue Hotel in the early 1850s for Madison Square (200 Fifth Avenue, on the southwest corner between 23rd Street and 24th Street), it was so far uptown that critics called it “Eno’s Folly.” When the hotel opened in 1859, Madison Square was becoming a center for New York City, and the hotel soon became one of the most famous and prosperous in the United States. Republican party bosses especially favored the Fifth Avenue Hotel, and a corner nook of a public room was dubbed the “Amen Corner.”
 
The name “Eno’s Folly” was printed in the book Lights and Shadows of New York Life: Or, The Sights and Sensations of the Great City (1872) by James Dabney McCabe. The Fifth Avenue Hotel was torn down in 1908 and the nickname “Eno’s Folly” is of historical interest today.
 
     
Wikipedia: Fifth Avenue Hotel
The Fifth Avenue Hotel was a luxury hotel located at 200 Fifth Avenue in Manhattan, New York City from 1859 to 1908. It occupied the full Fifth Avenue frontage between 23rd Street and 24th Street, at the southwest corner of Madison Square.
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The Fifth Avenue Hotel was built in 1856–59 by Amos Richards Eno at the cost of $2 million, was designed by Griffith Thomas with William Washburn. At the time of its construction it stood so far uptown from the centers of city life it was dubbed “Eno’s Folly”; New York bankers refused to capitalize the project, and Eno turned to Boston for funding.
 
The hotel, which quickly developed a reputation as New York’s most elegant, became “the social, cultural political hub of elite New York,” and brought in a quarter of a million dollars a year in profits.
 
Wikipedia: Amos Eno
Amos Richards Eno (November 1, 1810 – February 21, 1898) was an American real estate investor and capitalist in New York City. He built the Fifth Avenue Hotel and many other developments on the streets of Broadway and Fifth Avenue, where he established a prominent family fortune of 20 to 40 million U.S dollars.
   
Google Books
Lights and Shadows of New York Life:
Or, The Sights and Sensations of the Great City
By James Dabney McCabe
Philadelphia, PA: The National Publishing Co.
1872
Pg. 308:
The hotel was begun in 1857, Mr. Eno having more faith in the growth and prosperity of the city than most persons had at that day. The wise heads laughed him to scorn, and called his house ” Eno’s folly.” They said it might make a popular summer resort, but would never take rank as a first class city hotel. It was too high up town. Undismayed by these criticisms, Mr. Eno went on with his work, and in 1860, the marble palace, to which he gave the name of the Fifth Avenue Hotel, was opened to the public. By this time the city had grown so fast as to make the need of this house imperative, but the first years of the war laid a burden upon it which only the most skilful financial management could overcome.
 
22 February 1898, New-York (NY) Daily Tribune, pg. 7, col. 4:
AMOS R. ENO DEAD.
ONE OF THE LARGEST INDIVIDUAL OWNERS OF NEW-YORK REAL ESTATE.
Amos Richards Eno, one of New-York’s old-time merchants and one of the largest individual owners of real estate in the city, died at 11 o’clock yesterday morning at his home, No. 32 Fifth-ave.
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The foundation of Mr. Eno’s fortune was laid y hard work in the drygoods trade, but the bulk of it was acquired by subsequent real-estate investments. In 1854 he bought the land at Fifth-ave. and Twenty-third st. on which Barnum’s Hippodrome had stood, and began to build the Fifth Avenue Hotel. He was looked upon as little less than demented, and the building was frequently referred to as “Eno’s Folly.” The scheme, however, was believed in by Paran Stevens, then the foremost hotel-keeper in the country. Mr. Stevens leased the building, and in 1859 opened what has proved to be one of the most prosperous of American hotel enterprises; his associates and successors still continue his lease of the hotel.
 
Chronicling America
28 April 1918, The Sun (New York, NY), “The Story of The Sun 1833 to 1918” by Frank M. O’Brien, sec. 3, pg. 12, col. 2:
Amos R. Eno, merchant and banker, was the man who had made New York laugh by building the Fifth Avenue Hotel so far north—away up at Twenty-third street—that it was known as Eno’s Folly. This he did nearly ten years before Dana went to THE SUN, and in 1868 the hotel was not only the most fashionable in the United States, but the most profitable.
   
Ephemeral New York
July 27, 2015
The luxury power center of the Gilded Age city
When the white marble Fifth Avenue Hotel was set to open in 1859, it was mocked as “Eno’s Folly,” after the developer who built it.
 
With the city’s hotel district farther south on Broadway, why would anyone pay to stay on the outskirts of the city’s center, as Fifth Avenue and 23rd Street was at the time?
 
But after its grand opening, the Fifth Avenue Hotel became the city’s premier luxury residence and made Madison Square the focal point of post–Civil War New York.