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 September 11, 2001. Scorsese chose to end with that shot because the movie is supposed to be about the people who built New York, not those who tried to destroy it.
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.
Bill's hard "New Yok" accent wasn't entirely fabricated. Martin Scorsese 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.
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 Gaelic word ráibéad, meaning "man to be feared". "Dead" is a slang intensifier meaning "very." "Dead Ráibéad" thus means a man to be greatly feared.
Bill the Butcher is a rabid opponent of Abraham Lincoln. In one scene he throws a knife at a picture of the president. Day-Lewis would play Lincoln in Lincoln (2012) ten years later. Coincidentally Gangs of New York co-star Liam Neeson was originally cast to play Lincoln before Day-Lewis replaced him.
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 Scorsese and said, "Sets like that can be done with computers now."
At the Chinese theater, Bill the Butcher calls for his boys to play some "American music" and extols it as "patriotic." They play "Garry Owen," a Gaelic drinking tune which became the official song of the 7th U.S. Cavalry Regiment, chock-full of Irishmen. They were defeated, along with their commander, Gen. George Armstrong Custer, at Little Big Horn.
Bill says his father was killed by the British on July 25, 1814. He most likely died in the Battle of Lundy's Lane, which was fought on that date in the Niagara Falls area. It was the bloodiest battle of the War of 1812.
The initial battle between Cutting's gang and Priest Vallon's gang appears to have been based on an event that took place June 21, 1835, on Pearl Street between Chatham and Centre Streets, in the heart of 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 film is based partly on Herbert Asbury's book of the same name. The book's depiction of the draft riot, which remains the biggest in US history, is more in line with historical fact, and portrays the gangs as pro-slavery, racists, and lynchers.
During the final fight scene, Amsterdam wears a cestus, a Roman combat glove used in gladiator battles, on each hand. Cesti are essentially leather straps wrapped around the hands, but the Romans improved on the Greek design and added metal spikes, making them a more deadly weapon. Vallon's cesti are clearly visible when he prays before the fight.
In one scene, Boss Tweed describes to a few men the city's need for a grand new courthouse before being interrupted. It's a reference to the infamous New York County Courthouse, now known as the Tweed Courthouse. Tweed and Tammany Hall stole millions from the city that was earmarked for the construction of the building, making it the most expensive civic building of the 19th century.
Daniel Day-Lewis employed two circus performers to travel to his home in Wicklow, Ireland, to teach him how to throw thin, sharp daggers. He also went to work in a butcher's shop for several weeks to learn how to meticulously incise and gut carcasses.
Before the battle at the beginning of the film, several gangs introduce themselves. The Dead Rabbits, the Bowery Boys, and the Forty Thieves were real life New York gangs in the Five Points in the 1860s and 1870's. Their appearance, weapons of choice, and behavior are accurate. Many members became politicians later on.
Martin Scorsese said in an interview that he offered first the part of Bill "The Butcher" Cutting to Tom Hanks. He loved the script, but had to turn down the part due to his work in Road to Perdition (2002).
Production designer Dante Ferretti recreated over a mile of mid-nineteenth century buildings, consisting of a five block area of Lower Manhattan, including the Five Points slum, part of the East River waterfront with two full-sized sailing ships, a thirty-building stretch of lower Broadway and replicas of a mansion, Tammany Hall, a church, a saloon, a Chinese theater, and a casino.
The movie was originally planned for release around Christmas 2001. In June 2001, trailers and posters in theaters said "Christmas 2001" and "December." The film was pulled off the release schedule at the last moment, and released unchanged for Christmas 2002.
The film was conceived in 1978 and intended to be produced in 1980 or 1981. It was shelved after the box-office failure of Heaven's Gate (1980) made studios wary of expensive, ambitious historical dramas.
Book of Revelation, 3.14-16: "To the angel of the church in Laodicea write: These are the words of the Amen, the faithful and true Witness, the Originator of God's creation. I know your deeds; you are neither cold nor hot. How I wish you were one or the other. So because you are lukewarm-neither hot nor cold-I am about to spit you out of My mouth!"
Martin Scorsese) first encountered Herbert Asbury's 1928 book about the history of Five Points in 1970. His first call was to his friend Jay Cocks who would eventually collaborate on the screenplay over 30 years later. Scorsese told Cocks to "think of it like a western in outer space".
John C. Reilly was unsure about participating in the film when offered, but at the persuasion of friend and director Paul Thomas Anderson, a fan of Martin Scorsese himself, insisted to Reilly that he take the role. Reilly would later work again with Scorsese and Leonardo DiCaprio again in The Aviator.
Bill "The Butcher" Cutting states that his father died in 1814. In reality, William "Bill the Butcher" Poole, whom Cutting is based on, was born in 1821 and therefore his father could not have died during The War of 1812.
Priest's murder was originally much more violent. During the opening battle, just before Bill stabs Priest Vallon, 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.
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."