Warning: Contains minor spoilers for the His Dark Materials books.
His Dark Materials revealed a city in the sky, and in the season 1 finale Lyra Belacqua (Dafne Keen) and Lord Asriel (James McAvoy) are heading towards it - here's what you need to know about the mysterious location. Created by Jack Thorne (Harry Potter and the Cursed Child), the first season of His Dark Materials served as an adaptation of Phillip Pullman's classic novel The Golden Compass, aka Northern Lights. The eight-episodes followed Lyra and her daemon, Pantalaimon, as they ventured from Oxford to the Arctic in search of her friend Roger, and answers about the mysterious Dust.
The His Dark Materials season 1 finale saw the return of Lord Asriel (since revealed as Lyra's father), and unveiled his master plan. In order to learn more about Dust, and reach the city in the sky that he had first shown a photogram of in His Dark Materials' premiere episode, he would sever Roger from his daemon, using the energy unleashed to create a bridge to the new world. Lyra's unable to stop this from happening, but she does make a decision to go after her father into the city in the sky, which will now have a big role in His Dark Materials season 2.
The so-called city in the sky as seen through the Aurora Borealis is a place called Cittàgazze (meaning "City of the Magpies"), which exists within one of the many worlds that runs parallel to that of Lyra's. The seaside city, which as the name suggests is at least inspired by real-world Italian mercantile cities and towns, was once a thriving hub of trade and banking within its world. It was in this city in the sky that the Guild of the Torre degli Angeli was established, hundreds of years before the events of the series, who then founded the subtle knife (the same one that gives the second book in the His Dark Materials series its name).
The subtle knife, or Æsahættr, is used to open a gateway to other worlds. With the Guild increasingly interested in other worlds, they used the knife to try and explore the portals between them, letting in Spectres (creatures that feeds on Dust and, with that, human souls). These Spectres laid waste to the population of Cittàgazze, turning it from a thriving city into a desolate one. Adults fled because of the Spectres, meaning Cittàgazze's population is mostly children. The location with Cittàgazze that particularly stands out when seeing the city in the sky is the Torre degli Angeli, a tower in the main square which helps give the city so much of its distinctive iconography and shape. It's in this tower (or rather, on its roof) that Will Parry becomes the holder of the subtle knife.
With Lyra headed to the city in the sky, it means that season 2 of His Dark Materials will continue to follow the books quite closely. It's here in The Subtle Knife where she and Will first meet; at the end of His Dark Materials season 1, he too goes through a portal to another world, although it's left unclear if this is also to Cittàgazze or somewhere else. Considering the opening credits have teased Lyra and Will's meeting in Cittàgazze already though, and with both characters well-established, it seems likely that the city in the sky will be a key location in His Dark Materials season 2.