Alert: spoilers ahead.

The sixth episode of the latest season of Game of Thrones, Beyond The Wall, had a rough fight involving Jon and his raiding party and innumerable wights. One of the highlights of the episode was Daenerys joining the fight along with her dragons and the Night King killing one of the beasts, Viserion.

The episode involved a lot of computer generated imagery and practical effects, including turning the Wolf Hill Quarry in Belfast into an enormous icy set so as to combine the footage shot there with the parts shot in the wintry hell of Iceland. In a making-of video, the episode’s director, cast and crew take audiences behind the scenes.

Game of Thrones: The Frozen Lake.

Writers David Benioff and DB Weiss wanted to write a zombie polar bear into the series for the longest time, and they finally got the green signal from the special effects team in Beyond The Wall. Referring to a person wearing a green suit playing the bear on set, actor Rory McCann, whose character Sandor Clegane fights the bear onscreen, said, “I have had people going, ‘Oh, when I come towards you, I am a bear.’ No, you’re not ...You’re Toby.”’

A majority of the fight sequences was shot in the dry and greenish area of Belfast’s Wolf Hill Quarry, which was transformed into a temporary ice-land complete with a fake frozen lake.

HBO has also released the trailer for the 81-minute long seventh episode, which is the season finale. The trailer promises another battle sequence. Greyworm is shown to be leading the Unsullied into war, who are then joined by the Dothraki. Jon, Jaime and Cersei Lannister confront each other once, while Sansa looks on ominously in another scene.

Game of Thrones Season 7 finale preview.