Henderson Castle (also known as Henderson's Castle, Boundary Castle, and Prospect Castle) was a large Romanesque Revival house once located at 2200 16th Street NW in Washington, D.C. Built in 1889 for former Senator John B. Henderson and his wife, Mary Foote Henderson, the castle was sited on a prominent location overlooking the city. At the time, the area was mostly undeveloped, a fact Mary saw to change through decades of real estate development. Architect Eugene C. Gardner designed the castle, and later renovations were carried out by architects Thomas Franklin Schneider, George Oakley Totten Jr., and Laussat R. Rogers. Mary hired Totten to design many of the grand mansions and embassies on 15th and 16th Street NW which she sold to wealthy residents and foreign governments.

The Hendersons entertained prominent citizens at their castle. Mary was an advocate for women's rights, vegetarianism, and the temperance movement. Mary made national news when she and other members of the Independent Order of Rechabites took over 1,000 bottles of alcohol from the castle's cellar and smashed them on 16th Street. As part of her plans to develop the Meridian Hill neighborhood, Mary convinced Congress to buy land across the street from the castle, which was developed into Meridian Hill Park.

After Mary's death in 1931, the Hendersons' vast art collection and fine furniture were sold at auction. The castle remained unsold and it was converted into the Castle H Swim and Tennis Club in 1937. It was used as a boarding house and social event space until 1948, when all of the remaining contents were sold at auction. In January 1949, the castle was demolished. The property remained vacant for almost 30 years until Katharine Graham sold it to a real estate developer. A gated townhouse community, Beekman Place, was built on the site in 1976. Part of the castle's original perimeter wall and entrance gate still stands on 16th Street.

History Namesake John B. Henderson (1826–1913) was a lawyer and businessman from Missouri who served in the Missouri House of Representatives in the 1840s and 1850s. After the Civil War began in 1861, he was commissioned a brigadier general in the Missouri State Militia, though he never saw active service. In 1862, he was appointed to the United States Senate. During his time in the Senate, Henderson submitted a joint resolution for the Thirteenth Amendment to the United States Constitution, which abolished slavery. He gained a reputation as an independent voice in the Republican Party, often bucking party leadership evidenced by his vote against the impeachment of President Andrew Johnson. For this action, he did not run for re-election and left office in early 1869. Henderson returned to Missouri where he continued practicing law and became wealthy through real estate and bond speculation.

While still serving in the Senate, Henderson married Mary Foote (1842–1931), daughter of Elisha Foote, a lawyer and judge, and Eunice Newton Foote, a women's rights campaigner and scientist who was the first person to describe what is now known as the greenhouse effect. After John and Mary returned to Missouri, Mary authored books on healthy living and hosting social events, studied art at the Washington University in St. Louis, and helped establish the St. Louis School of Design. As a proponent of women's suffrage, she co-founded the Missouri State Suffrage Association. Mary was known as a strong-willed woman who supported women's rights, vegetarianism, and the temperance movement.

House history Construction The Hendersons returned to Washington, D.C. in the 1880s after John retired. While searching for suitable land to build their house, the couple lived at the Arno Hotel on 16th Street NW between I and K Streets. North of their hotel, an area beyond the original city boundaries had remained mostly undeveloped. This area, named Meridian Hill, was north of Boundary Street (present-day Florida Avenue). Between January 1887 and April 1888, the couple purchased six lots in Meridian Hill from A. P. Fardon, James Curtis, and William C. Hill at a total cost of almost $50,000. John and other area landowners formed the Meridian Hill Improvement Association which sought funds from Congress to make improvements in the neighborhood.

On April 12, 1888, a building permit was issued for the Henderson's new house, located just south of the Joaquin Miller Cabin. Mary played a large role in overseeing its design and construction. The couple's choice of architect Eugene C. Gardner was possibly due to Gardner's 1882 book The House That Jill Built, After Jack's Had Proved a Failure, which told "how a wife's clearheaded analysis of a house's functions overcame her husband's dysfunctional design preferences." Gardner was from Springfield, Massachusetts, but briefly ran an office located at 1420 New York Avenue NW in Washington, D.C. in the late 1880s. His designs, many of which are now historic landmarks, included churches, schools, houses, hotels, and other commercial buildings. There are fifteen known buildings in Washington, D.C., designed by Gardner, including a now-demolished house for Stilson Hutchins, founder of The Washington Post.

The total cost for construction of their house, located at 2200 16th Street NW, was around $50,000, bringing the total cost of the land and house to approximately $100,000. () The red Seneca sandstone house, completed in 1889, was designed in the Romanesque Revival and supposedly modeled after a castle Mary had seen in Germany or Normandy. The main portion of the house was three-stories and there were two four-story towers. Due to its medieval appearance and location just north of Boundary Street, the house was known originally known as Boundary Castle. It was later referred to as Henderson Castle, Henderson's Castle, and Prospect Castle. The entrance gates were located on 16th Street, but the castle faced south towards the city. According to historians Sue A. Kohler and Jeffrey R. Carson, "the location and design gave the impression that the castle was somehow strategic to the defense of the city, while the heights gave the occupants a psychological advantage over the ferret-like maneuvering of those who resided below." It would earn a reputation as one of Washington's most famous Gilded Age houses.

Early years On February 10, 1890, the Hendersons hosted their first formal dinner at their castle. The invited guests included numerous delegates in town for the First International Conference of American States, justices of the Supreme Court, members of Congress, ambassadors, generals, and other members of high society, including Andrew Carnegie and John George Nicolay. The great ballroom was 100 feet (30.4 m) long, 75 feet (22.9 m) wide, and 30 feet (9.1 m) high, and included a stage and balcony. A description of the house interior was provided by a reporter from The Evening Star:

After the success of this event, many were eager for an invitation to dine at Henderson Castle, even though Mary made sure to only serve vegetarian cuisine. Two years later, the Hendersons hired local architect Thomas Franklin Schneider to design a stable for ten horses and a large wing for servants' quarters. The wing was decorated with crenelated battlements, further giving the house a castle-like appearance. The cost of construction was around $20,000. In 1897, a two-story brick and stone addition with basement was built. The architect is unknown, and the cost was around $18,000.

Inspired by the City Beautiful movement, Mary sought to transform 16th Street and Meridian Hill into a prominent gateway into the nation's capital, similar to the Champs-Élysées in Paris. In 1898, there were suggestions the White House was too small, and a larger, grander one should be built. Mary worked with architect Paul J. Pelz to design a new executive mansion to be located in present-day Meridian Hill Park, across the street from the castle. Congress declined her suggestion. Two years later, she worked with architect Franklin W. Smith on another design, which was also rejected by Congress. In 1901, a brick garage was added to the castle "wherein to keep a locomobile", later expanded to accommodate four vehicles. The following year, the couple hired architects George Oakley Totten Jr. and Laussat R. Rogers to remodel the castle. This $8,000 project entailed replacing iron cornices and battlements with stone and replacing the three-story wooden bay portico with stone masonry. After purchasing two adjoining lots in 1903 from Mary Ord Preston and James Orme, the total area of their estate on 16th Street was .

The Hendersons bought lots in the surrounding area, the beginning of a long history of Mary investing in local real estate. She eventually owned around 300 lots and played a large role in the development of Meridian Hill. Having been impressed with Totten's work, Mary worked with him on designing nearly a dozen palatial embassies and residences along 15th and 16th Streets. The first was the Pink Palace, built in 1906. That same year Mary and other members of the Independent Order of Rechabites, a temperance movement, caused an incident when the women decided to dispose of almost 1,000 bottles of wine, whiskey, and brandy from the castle cellar. As reported in a front page article of The New York Times, after bringing the bottles to 16th Street, the women smashed them, creating a river of alcohol flowing down the road. Two months later, it was reported there was a congressional proposal to convert the castle into the presidential residence at a cost of $600,000, but the cost was deemed excessive by the Board of Commissioners.

After the earlier unsuccessful attempts to have the new presidential house built across the street, in 1910 Congress agreed to pay $490,000 for the of property for use a public park. Two years later, Mary asked Congress to build the proposed Lincoln Memorial on the site. She had hired architects Frederick V. Murphy and Walter B. Olmsted to design her proposal, but as was the case with the new White House, Congress rejected her offer. The following year John died after suffering from a long-term kidney illness. Mary continued her real estate business, selling lots to individuals who promised to build lavish homes, and working with Totten on his designs of the French, Lithuanian, Polish, Cuban, and Spanish embassies. In 1914, Mary had a pavilion and tennis court constructed on her property, followed by an extension of the brick wall running along Florida Avenue. She had a swimming pool built in 1921, one of the first private pools in the city. Meanwhile, construction continued on Meridian Hill Park, and she often had to lobby Congress for its funding.

Later years During an event held at the Japanese embassy around 1920, Mary and her son, John B. Henderson, Jr., met Jesse Shima, an immigrant from Okinawa who had recently moved to Washington. Impressed by his demeanor, Mary invited him to work for her at the castle. After initially declining her offer, Mary sent her limousine to pick him and bring Shima to her home. She hired Shima to assist in the castle and paid for him to attend National University School of Law for three years and to take pilot lessons with Mount Vernon Airways. When Henderson Jr., her only surviving child, died in 1923, Mary asked Shima to oversee her estate office. The two became close, with Shima acting as her personal assistant and constant companion. The two ate together and Shima slept in her son's former room. Shima oversaw all staff operations at the castle, including sending the gardener to collect trash on 16th Street all the way to the White House, upon Mary's insistence. According to Shima, Mary showed him a secret compartment in her dressing room where she kept a cache of diamonds and other jewelry.