Darkar's Fortress

Darkar's Fortress (also Shadow Castle or, in the 4Kids dub, Shadowhaunt) was Lord Darkar's fortress in Season 2 and the home for the Trix as well.

Overview
Darkar's fortress was an underground fortress which was difficult to enter, let alone approach, because of the intense negative energy found there, as well as the lack of sunlight, weakens creatures living on the surface and also because it was guarded by the Shadow Monsters of the Under Realm were under Darkar's command. Because of this even Bloom, despite possessing the Dragon's Flame, the most powerful power in the Universe, was weakened when she went to Darkar's Fortress to save the Pixies.

Although the fortress has been seen only in Season 2, it is unknown for how long has it been existing and whether Lord Darkar was already residing there before he was defeated by the Lord of the Knights of Roccaluce.

Season 2
At the beginning of this season, Darkar kidnapped the Pixies and imprisoned them in his fortress in an attempt to find the location of the Codex. Aisha tried to rescue the Pixies but she was stopped and defeated by Lord Darkar, and she only managed to save Piff.

After Aisha arrived at Alfea, Faragonda sent her together with Bloom, Stella, Sky and Brandon to rescue the Pixies. After many difficulties and passing through the realm of Downland, they managed to save the Pixies with the help of a mysterious Paladin who was later revealed to be the new teacher at Alfea, Professor Avalon.

Towrds the end of the season the fake Avalon who had been sent to Alfea by Darkar kidnapped Bloom took her to Lord Darkar who turned her into Dark Bloom to use her to obtain the ultimate power of the Relix. The remaining Winx and Specialists then launched an assault on the castle and were soon followed by two of the school directors and a Red Fountain teacher (Saladin sent Codatorta in his place). After Darkar was defeated in the Relix Realm, most of Darkar's fortress was left in ruins.

Season 3
Stormy suggested to Valtor that they should move to Lord Darkar's Fortress, Darcy rejected the idea because she does not want to live "underground like a mole."

Season 2
Darkar's Fortress is first made mention of in Issue 19: Monsters on the Loose as the Pixies recount their latest misadventures to the Traveling Pixie with their picture book.

With everyone settled at the Story Tree, Ninfea opens the book to a drawing of the Fortress as she explains to the Traveling Pixie that a majority of their friends were kidnapped by Lord Darkar and imprisoned within the Fortress alongside Aisha, a dear friend of theirs, so that he could learn the location of their village. With enough prodding, Ninfea tells the Traveling Pixie that Darkar wished to learn the location of Pixie Village to steal its quadrant of the Codex, which would bring him one step closer to ultimate power.

Once Chatta and Tune were done bickering over who could tell the rest of the story, Ninfea continues where she left off by telling the Traveling Pixie of Aisha's bravery as she climbed the side of Darkar's Fortress in an attempt to free her Pixie friends after breaking free from her own imprisonment. Unfortunately, Darkar caught Aisha and knocked her off the Fortress and into a river, though, Aisha would later return to the Fortress with the Winx Club, her new friends, and the Specialists. Despite all the odds being stacked against them, the group would rescue the Pixies, bond with them, defeat Darkar and the Trix, resulting in the Fortress' destruction.

Long after Darkar's defeat, groups of students from Alfea, Cloud Tower and Red Fountain would have to return to the mountains above where Darkar's Fortress was located to ensure that all the Shadow Phoenix's monsters were destroyed along with him.

In the following issue, it is revealed that the Trix survived the cave-in caused by the destruction of Darkar's Fortress and now live within its remains as highly wanted fugitives. Wanting to reintegrate into society to escape having to live like rats, Icy eventually comes up with the idea to erase the memories of everyone in Magix so that no one would be able to remember their past misdeeds. The Trix find just the spell they are looking for after cracking open a book Icy smuggled from libraries of the Fortress and quickly leave the Fortress remains to put Icy's plan into action.

Trivia

 * Lord Darkar's fortress might have been partly inspired from Barad-dûr, the fortress of the Dark Lord Sauron from The Lord of the Rings by J. R. R. Tolkien, and by Mount Doom, the volcano found near Barad-dûr.
 * In The Lord of the Rings, Sauron had poured a large amount of his power into Barad-dûr and Mount Doom such that, when Sauron was defeated for good, Barad-dûr collapsed (while in earlier times the enemies of Sauron had been unable to completely destroy Barad-dûr because Sauron was still alive) and Mount Doom had a massive eruption after which it became dormant forever, similar to how Darkar's fortress and all the caves around it collapsed after Darkar was defeated.
 * In The Lord of the Rings, the darkness at the heart of Mount Doom was so intense that even the light of the Phial of Galadriel, containing the light of Eärendil's star, the purest light of the universe of the Lord of the Rings, was subdued and Frodo's will was broken and he ended by becoming corrupted by the One Ring. This is similar to how the evil at the heart of Darkar's realm was so powerful that even Bloom, who possessed the Dragon Flame, the most powerful power of light in the Magic Dimension, was severely weakened.
 * The realm of Darkar may also have been partly inspired by Mordor, the land ruled by Sauron. Mordor is a land peopled by evil creatures such as orcs, trolls and giant spiders and where normal life cannot exist.
 * Utumno, the fortress of Morgoth, the original Dark Lord and Sauron's master in The Silmarillion, might also have been an inspiration for Darkar's fortress as both are very deep underground fortresses made of many caves, tunnels, corners and rooms.