Fortress of Light

The Fortress of Light, also known as the Monastery of Roccaluce or Lightrock Monastery in both the Cinélume dub and the Nickelodeon dub, is a sacred temple found in Magix.

Overview
The Fortress of Light hangs above the Lake Roccaluce, in non-space (nobody knows where it is exactly) on a huge rocky mountain and immersed in the clouds. You can only reach it by traveling through the dimensions and you need to be very well versed in magic to do that. The temple is stately and imposing and has magnificent exterior walls. There is a purification cell inside for those who have committed serious crimes. The "cell" is more purifying and redemptive than punitive. When the person reaches the right spiritual level, they may cross the threshold without encountering any obstacles as long as they no longer pose a threat.

The Knights of the Fortress of Light live here, the guardians of the temple. They study "Wu-Gong" here, that is an ancient martial art that leads to the development of very highly qualified, magic-resistant warriors

Pre-Series
The Lord of the Knights is the one who defeated and banned Darkar shortly after the destruction of Domino, sending him into a 17-year long sleep of which he awoke in the beginning of Season 2, and he and his Knights helped the Company of Light fight the Ancestral Witches according to the official site prior to the Season 3 revamp. He is likely the third force that Wizgiz spoke of that only appear in special circumstances.

After the Trix were defeated at the end of Season 1 they were sent to LightRock Monastery by the Knights at the request of Headmistress Griffin.
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In the beginning of Season 2 the Trix were freed from the Fortress and were given new Gloomix powers, which give them the ability to defeat the Knights, by Darkar.
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After Lord Darkar abducted Bloom in the second half of the season, Faragonda stated that the Lord of the Knights and his Knights should never be involved in the fight between light against Darkness.

Earlier in the season. Professor Wizgiz said that when the balance between Light and Darkness is disturbed in the Magic Dimension, a third force interferes to bring back the balance. This might mean that he will only join the fight if the Light and Darkness balance is seriously jeopardized, like the war 16 years prior to the series, and if the Magic Dimension is on the brink of destruction.

After the events in the Realm of Relix at the end of Season 2, the Trix were sent to the Omega Dimension instead of the Fortress of Roccaluce since they were considered as being too dangerous for the Magic Dimension.
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There, they freed themselves thanks to Icy's ice powers and found Valtor with whom they allied themselves. Valtor later attacked the Fortress of Light itself in is quest to steal and obtain all the magical secrets and powers of the Magic Dimension.

When Valtor lost all the spells he stole towards the end of Season 3, the Trix left him and tried to flee, but were captured again by the Knight-Monks of the Fortress of Light, who however predicted to the Winx that they will have to face the Trix again one day.

Magical Adventure
The Trix reappear in the second film, though it is unclear how they escaped from the Fortress of Light.

At the end of the film they are captured by the Winx, presumably to be handed back to the Knights of the Fortress of Light.

At the beginning if Season 5, the Trix are imprisoned in the underwater Prison of Andros, from where they are freed by Tritannus. It is presumed that they were sent there by the Knights of Roccaluce.
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Trivia

 * Codatorta was once a Knight of the Fortress of Light.
 * The Knights of Roccaluce are responsible for punishing the criminals of the Magic Dimension and it is assumed that they have a very high level of authority throughout the Magic Dimension and that they are in charge of the Omega Dimension.
 * It might be possible that the Knights of Roccaluce are the ones who captured the Trix from the Relix Dimension and sent them to the Omega Dimension. They might either have entered the Relix using their advanced magical skills or by using a copy of the Codex they might have owned.
 * In the comic series the Knights are alerted when the Wizards of the Black Circle escape from the Omega Dimension.