Frost Tower

The Frost Tower (or the Ice Tower) is a floating mass of ice and icy spires that flies around the Earth. It is under the ownership of Aurora, Major Fairy of the North, and acts as her home, as well as the home of her followers, the Arctic Fairies.

Exterior
The Frost Tower is a large floating ice mass with multiple spires sticking out from it; the largest of which holds Aurora's throne at the very top. The Tower can also withstand various climates ranging from space or high atmospheric areas to areas closer to the Earth's surface without any issues. The Tower also holds a balcony around its highest point that can act as an arena of sorts like in the case with Bloom's duel against Nebula in the episode "Bloom's Trial."

Interior
The inside of the Frost Tower is made up of a large maze of pathways that lead to every possible direction and, since it is under Aurora's ownership, she can control where these pathways lead as well as the Tower's flight path, speed, internal temperatures and so on.

At the very top of the Frost Tower lies its throne room that acts as both Aurora's private quarters and a vault for the Blizzard, the source of the Major Fairy's ice powers. The rest of the Frost Tower also acts as a home to Aurora's faithful followers, the Arctic Fairies.

Season 4
The Frost Tower makes its debut in the episode "The Frozen Tower," where Aurora, Major Fairy of the North, unleashes a fearsome freezing spell onto the Earth under Earth Fairy High Queen Morgana's orders after catching word of the Winx's latest success, as well as Diana's decision to defect from their Age of Vengeance.

Using the Gift of Heart given to them by the Ethereal Fairies, the Winx transform into their newfound Lovix forms (excluding Roxy) and make their way over to the Frost Tower to confront Aurora, hoping to convince her to stop the freezing spell and give up her vengeance as well.

For most of the episode, the Winx brave through the Frost Tower battling the Arctic Fairies until they arrive in the throne room, where Aurora had been waiting with her power source, the Blizzard, in hand. Aware that her powers would be no match for the Dragon's Flame that Bloom wields, Aurora instead traps the Winx within a cage of frost and snow as she teleports outside of the Tower, leaving them to freeze to death. Morgana and Nebula also arrive by the Frost Tower to check on Aurora's progress.

Roxy is the first to succumb to the harsh cold of the Tower, which prompts Bloom to strike a deal with the Earth Fairies to ensure her safety, as well as the safety of all human life on the planet. Leading into the following episode, "Bloom's Trial," Bloom proposes that she be pit against the strongest warrior the Earth Fairies have to offer and, should she win, they must give up their Age of Vengeance and dispel of the freezing spell that is still placed over the planet. So, at the highest point of the Tower, Bloom does battle with Nebula, Major Fairy of War, and wins just by the skin of her teeth, which prompts Aurora to do away with the freezing spell, also allowing for Roxy to regain consciousness. However, Nebula, blinded by her thirst for revenge, refuses to honor the deal and tries to crush a portion of Gardenia by throwing down one of the Tower's icy spires. Bloom is barely able to destroy the spire before it hits the city but exerts herself to the point of exhaustion and cannot fly back up to the Tower. Luckily, Roxy is able to catch and bring her back before reprimanding Morgana for letting this happen (therefore not sticking to the arrangements of the deal), which causes Morgana to ultimately denounce the Age of Vengeance. However, all but Nebula and a few other Earth Fairies agree with their queen's decision.

Trivia

 * The Frost Tower may have been inspired by the flying chariot that the Greek Goddess, Eos, uses to fly across the sky and signal the coming of her brother Helios, God of the Sun.