Sigur Rós, an Icelandic post-rock band, was hired as the wedding band for King Joffery's wedding ceremony. You can listen to their version of 'The Rains of Castamere' at the end credits of the episode, and witness the band itself at the wedding; the three of them are those whom the king throws coins at.
The episode title refers to the wedding of Joffrey and Margaery, and the sigils of their houses: Joffrey is a member of Houses Baratheon of King's Landing (stag) and Lannister (lion), and Margaery is a member of House Tyrell (rose).
The first appearance of Dean-Charles Chapman as Tommen Baratheon. Callum Wharry previously played the role in the first two seasons. Chapman first appeared on the series as Tommen's cousin Martyn Lannister in season three.
This is the last episode of the series written by George R.R. Martin. Martin had intended to write an episode for every season, but stopped to concentrate on writing the sixth "A Song of Ice and Fire" novel, "The Winds of Winter".
When Joffrey asks for suggestions for the name of his new sword, the first name shouted out is "Stormbringer". This is the name of the sword of Elric of Melnibone, and the name of the first novel of a seminal fantasy series by writer Michael Moorcock about this demon-ridden anti-hero. Another suggestion is "Terminus", perhaps a reference to the sword wielded by Severian, the protagonist of Gene Wolfe's "The Book of the New Sun" novel series.
The scene in which Joffrey asks Tyrion to get his drink just before he takes his fatal sip had to be re-shot several times. Peter Dinklage and Lena Headey kept laughing when Jack Gleason said the line "Hurry up, this pie is dry".
The trivia items below may give away important plot points.
Joffrey's wedding has been nicknamed by the fans as "Purple Wedding", referring to the wine used to assassinate Joffrey and the amethysts on Sansa's special hairnet (changed to a necklace of blue stones in the show), as well as to draw a parallel with the infamous Red Wedding. The nickname has become so popular that even George R.R. Martin and the HBO production team have been known to use it. The term is never used in the novels, though.
According to George R.R. Martin, Joffrey's death during his wedding feast was inspired by the death of Prince Eustace of Boulogne in 1153, who died from choking on his food at dinner, though some historians believe poisoning was the actual cause of death.
In the novels, Jaime hasn't made it to King's landing yet with Brienne, thus did not attend the wedding. He heard about Joffrey's death on the road to the city, and was totally indifferent, thinking that Joffrey deserved to die.
Violet (Stephanie Blacker), the girl who seduced Theon together with Myranda in Game of Thrones: The Bear and the Maiden Fair (2013), may have been the originally intended victim of Ramsey's dogs in this episode. George R.R. Martin claimed he had a scene written for her for the fourth season, but Blacker was unavailable due to her pregnancy. The character of Tamsy may thus have been created in her place.
In the books, Axell Florent is Selyse's uncle, not her brother. It is another of Selyse's uncles, Alester Florent, whom Stannis has burned alive, and not for refusing to renounce the Faith of the Seven but due to alleged treason.