Not familiar with the name? I don’t blame you. Neck Quarter was the former name of a parcel of land located in King George County, Virginia owned by Nathaniel Hooe. In December of 1845, Hooe sold this tract of land to Dr. Richard H. Stuart. His wife subsequently renamed Neck Quarter to its modern name, Cleydael.
Dr. Stuart was one of most prominent doctors and wealthiest men in the county. Before buying Cleydael’s land, Dr. Stuart owned land and a house eight miles away near the coast of the Potomac called, “Cedar Grove”. While Dr. Stuart and his family enjoyed Cedar Grove, the hot, muggy, summers near the Potomac proved unpleasant with cases of malaria being common. Upon purchasing Neck Quarter from Nathaniel Hooe, Dr. Stuart began construction on a summer home. This summer home utilized an unusual design that created cross breezes to naturally cool the house during the hot summers. When the Civil War began, Dr. Stuart and his family left Cedar Grove and began residing at Cleydael year round. Their home on the Potomac was deemed too dangerous as the threat of Union shelling was a very real one. During the war, Cleydael would house General Robert E. Lee’s daughters (cousins to Mrs. Stuart) when they were forced to leave their home at Arlington. Dr. Stuart continued his practice from the safety of this home. An office with a waiting room, and easy passage between it and Dr. Stuart’s bedroom, allowed the good doctor to continue to service patients even late at night.
On the night of April 23, 1865, John Wilkes Booth and David Herold were led to Dr. Stuart’s by Confederate agent, William Bryant. The doctor, having heard about Lincoln’s assassination was suspicious of the men and refused to let them stay. He relented to giving the men a meal before sending them on to the cabin of William Lucas, a free black who lived nearby. Booth would later write a poison pen thank you letter to Dr. Stuart for his “generosity”. While Dr. Stuart would spend a month in prison, it was this letter that proved his innocence and refusal to help Booth.
So where does the name Cleydael come from anyway? Mrs. Stuart’s maiden name was Julia Calvert. She was the granddaughter of Henri Stier, a wealthy Belgian baron. Her grandfather’s home was Château de Cleydael near Antwerp, Belguim:
When the French army invaded Antwerp in 1794, the baron and his family fled to America, leaving Cleydael behind. Mrs. Stuart renamed their summer residence Cleydael in honor of her ancestral home.
Recently, there was much worry over the future of Cleydael. The previous owner passed away without a will and with debts to be paid. Despite a historical easement on the house, there was a real chance the house and property would be sold and demolished. Luckily, such a crisis was averted when the house was recently bought by a couple committed to restoring and retelling the history of Cleydael.
Resources:
Cleydael’s 1937 Virginia Historical Inventory Project record
Cleydael’s 1986 National Register of Historic Places Nomination form (.pdf)
LincolnConspirators.com 
You mentioned that Mrs. Dr. Stuart was Julia Calvert. In one of the strange twists in American history, Julia’s brother was George Calvert, whose letter demanding final payment from Mary E. Surratt on property that her husband had purchased in 1852, brought the ill-fated widow to her country home in Surrattsville on the afternoon of April 14, 1865. At that time, she delivered Booth’s field glasses and instructions to have the “shooting irons ready” for parties that would call that night. Fugitives Booth and Herold were the parties en route into Southern Maryland after shooting Lincoln.
I am a descendant of Nathaniel H. Hooe.