Martin Scorsese ends the film with a shot of the New York skyline which includes the World Trade Center Towers, even though the film was finished after the buildings were destroyed in the terrorist attacks of 11 September 2001. Scorsese chose to end on that shot rather then continue with a skyline without the WTC because the movie is supposed to be about the people who build New York, not those who tried to destroy it.
Bill the Butcher (played by Daniel Day-Lewis is a great opposer of Abraham Lincoln and in one scene he's seeing throwing a knife on a picture of the president. Ironically, Day-Lewis would later play Lincoln in Lincoln (2012).
Most of the gangs mentioned by name were real 19th century New York gangs. Bill "The Butcher" Cutting is based largely on real-life New York gang leader Bill Poole, who also was known as "The Butcher" and had much the same prestige as Daniel Day-Lewis' character.
At one point in the film, Monk (Brendan Gleeson) speaks a line of Gaelic to Amsterdam (Leonardo DiCaprio) and then translates it. Before working as an actor, Gleeson taught Irish Gaelic (among other subjects) as a secondary school teacher.
The name "Dead Rabbits" has a second meaning rooted in the Irish American vernacular of 1857. The word "Rabbit" is a phonetic corruption of the Irish word ráibéad, meaning "man to be feared". "Dead" is a slang intensifier meaning "very." "Dead Ráibéad" thus means a man to be greatly feared.
When Boss Tweed is talking to Bill, Bill says to him "I know your works. You are neither cold nor hot. So because you are lukewarm, I will spew you out of my mouth." though never mentioned, this is a direct quote from The Holy Bible; Revelations 3:16
Bill says his father was killed by the British on 25 July 1814. This was probably in the Battle of Lundy's Lane, which was fought on this date in the Niagara Falls area, and was the bloodiest battle in the War of 1812.
Bill's hard "New Yok" accent wasn't entirely fabricated. Martin Scorsese actually did some research by listening to a voice recording of Walt Whitman and by reading an old play in which the dialog was spelled out phonetically.
During the boxing scene, there is a cutaway to a man drawing a caricature of "Boss" Tweed. This is a reference to the political cartoonist Thomas Nast, who was primarily responsible for the eventual downfall of Tweed.
The film is based partly on Herbert Asbury's book of the same name. In it the depiction of the riot (which is still the largest in US history) is more in line with historical fact, which portrays the gangs as pro-slavery, racists, and lynchers.
The initial battle between Cutting's gang and Priest Vallon's appears to have been based on an actual event that took place on June 21, 1835 (ten years earlier than depicted in the film) on Pearl Street between Chatham and Centre Streets, which is in the heart of the Five Points. The New York Sun wrote of "a most disgraceful riot" whose origin "was a dispute between two native citizens and several foreigners." According to the paper's account, "the riotous assemblage amounted to several thousand (people), many of those concerned armed with stones, brickbats and bludgeons."
The POV shot where Amsterdam re-emerges into the Five Points after recuperating from his wound (specifically, the four or five men loafing on either side of the alley) is a visual reference to a Jacob Riis photo, "Bandit's Roost," used as cover art on some editions of Herbert Asbury's "Gangs of New York."
Many of the characters portrayed in the movie are actually buried in Green-Wood Cemetery in Brooklyn. The view of the skyline shown at the end of the movie would not be visible from this location, but rather from the Williamsburg section of Brooklyn.
In one scene Boss Tweed is describing to a few men the city's need for a grand new courthouse before being interrupted. This is a reference to the infamous old New York County Courthouse, now known as the Tweed Courthouse, where Tweed and Tammany Hall had stolen millions from the city that was earmarked for the construction of the building, which became the most expensive civic building of the 19th century because of Tammany's theft of funds.
Martin Scorsese recreated 19th-century New York on the lot of Cinecitta studios in Rome. When George Lucas visited the massive set, he reportedly turned to Martin Scorsese and said "Sets like that can be done with computers now."
During the scene at the Chinese theater, Bill the Butcher calls for his boys to play some "American music" and extols it as "patriotic." The tune they play is "Garry Owen," a Gaelic drinking tune, which became the official song of the 7th U.S. Cavalry Regiment, chock full of Irishmen and infamous for their defeat, along with their commander, Gen. George Armstrong Custer, by Indians at Little Big Horn.
During the final fight scene, Amsterdam is shown wearing a cestus on each hand. A cestus is a Roman combat glove used in gladiator battles. They are essentially leather straps wrapped around the hands, but when the Romans improved on the Greek's design and added metal spikes, they became a more deadly weapon. You can clearly see Vallon's cesti when he is praying before the fight.
The movie was originally planned for Christmas 2001 release. In June 2001, trailers were released in theaters along with posters being displayed with "Christmas 2001" and "December" listed on them. At the last moment the film was pulled off the release schedule. It was released unchanged for Christmas 2002.
The film was conceived in 1978, and intended to be produced sometime in 1980 or 1981, but the box office failure of Heaven's Gate (1980) made studios wary of expensively ambitious historical dramas, so the idea was shelved.
Priest's murder was originally much more violent. During the opening battle, just before Bill stabs Priest Valon, an axe severs his left arm at the elbow, then Bill hacks him limb from limb. The shot of the arm being severed is still in the film moments before Bill yells for Priest to turn around.