Q. What is it supposed to be on the show?

A. A castle-like mansion and its gardens.

 
Q. Where is it supposed to be on the show?

A.  The location isn't specified, only that it is within a helicopter ride of New York City. 
 
So while it is likely that they were probably just flying to somewhere on Long Island, given that a civilian helicopter has a typical range of around 250-400 miles, they theoretically could have reached cities as distant as Richmond, Virginia.

Q. When did we see it on the show?

A. We see this estate in Episode 7 of Season 1 of Resurrection. It is where billionaire Leon Prater retreats for serial killers.
 
After Dexter meets Leon Prater at his helicopter pad, he joins the rest of Prater's bizarre group as the copter flies them to a special retreat at what Prater refers to as “the castle”, a large mansion that does indeed resemble a castle, and which features lush, formal gardens.

Prater tells Charley that he “leased the castle for the entire day”, so apparently he doesn't own the estate, he simply uses it for these eccentric social events. But he also mentions that Mia (“Lady Vengeance”) loved her first visit there, four years ago, indicating that he probably leased the castle on a fairly regular basis.

After telling the group that Mia killed herself in prison, Prater leads them in a toast to her memory. Dexter wonders if she actually killed herself, or whether Prater might have had her silenced.

After spending some time in the garden, the retreat moves on to a major buffet meal, where Dexter (still impersonating the Dark Passenger killer) decides to drop his pretense of being a vegan, and enjoy the expensive meats in the lavish spread. 

Then they move on to semi-mandatory presentations. “Rapunzel Al” shows the group a repulsive home video of him stalking and strangling a girl jogger, then cutting off her ponytail as a trophy.

When it’s Dexter’s turn, he decides to dispense with his original plans to mimic Ron Schmidt, and instead delivers a fairly honest monologue in which he talks about what it’s like to live under the control of a “dark passenger”.

His speech so impresses Leon Prater, that the billionaire opens up to Dexter and reveals his own origin story, which involves his parents being killed by a murderous driver, followed by him developing a peculiar relationship with that imprisoned criminal, leading to his lifelong obsession with talking to serial killers.

Later Dexter decides to eliminate the remaining Gemini twin, and to do so right in front of the others.

So he sits next to Gareth, within sight of the group, and then goads Gemini into attacking him with a broken wine glass, which Dexter then uses to kill Gareth, making it look like a case of self defense.

This bloody death so upsets Al that he demands to go home, and the next time we see the group, they are landing back in New York in the helicopter

 
Q. What is it actually in real life?

A. A castle-like mansion and its gardens.

Q. Where can I find it in real life?

A. This is reportedly the second largest home in the United States.

Known as Oheka Castle, with the official address of 135 W Gate Dr, Huntington, New York, it is about 10 miles east of the Guggenheim Mansion, as the crow flies. ( But if you are actually driving the roads, it is more like 20 miles.)

 

This “castle” has more than one thing in common with the Guggenheim mansion (which was used for the interior shots of Leon Prater's mansion): Not only are both of these mansions on the Gold Coast of New York's Long Island, and not only were both built around the turn of the century by tycoons, but like the Guggenheim Mansion, this castle is also now located inside a golf country club, this one called the “Cold Spring Country Club”. (The official address of the Country Club is 22 E Gate Dr, Huntington, New York.)
 
So both of these grand estates ended up surrounded by golf courses.

Oheka Castle, was built between 1914 and 1919 by Otto Hermann Kahn, a prominent investment banker who specialized in the reorganization and consolidation of major American railroads like the Union Pacific. The name “Oheka” derives from his initials: OHK.
 
The castle is apparently now a hotel and a wedding venue, and offers public tours.

You can find its website at: https://www.oheka.com

 The castle is the second-largest private home in the United States, originally comprising 127 rooms and over 109,000 square feet.

The paneled room in which Dexter gives his presentation is known as the Addie Kahn Room & Library:

 
 
At first, I thought that the castle grounds & gardens were going to provide the exterior scenes of Prater's Manhattan mansion, while they used the Guggenheim mansion for the interior shots of Prater's mansion.  But it turned out that it was used as a completely separate estate. Although Prater leased the estate on a regular basis, he did not own it.

And as a result, in addition to the exterior scenes, they were able to also use the interior of the castle as well, for the scenes of the lavish buffet, and the scenes of Al's and Dexter's morbid presentations, since the two mansions have very different interiors.


  Here is a matching 360° StreetView Of Oheka Castle:
 

 
Here is an aerial photo of the location.  And here is a map link.