The sacred temple of the Fortress of Light, also known as the Monastery of Roccaluce or Lightrock Monastery in the Nickelodeon dub, hangs above the Lake of the Fortress of Light, 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
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.
Faragonda also stated that he should never be involved in the Light-Darkness fight after Darkar abducted Bloom. 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 the Magic Dimension is on the brink of destruction.
The Trix were sent there at the end of Season 1, only to be freed in the beginning of Season 2 by Darkar. After the events in the Realm of Realix at the end of Season 2, the Trix were sent to the Omega Dimension since they were considered as being too dangerous for the Magic Dimension.
There, they freed themselves thanks to Icy's ice powers and found Valtor with whom they allied themselves. 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. The Trix reappear in the second film, though it is unclear how they escaped from the Fortress of Light.
Together with their ancestors, the Ancestral Witches, they disturb the balance between Good and Evil of the Universe, only to be defeated again by the Winx who destroy the Ancestresses for good. 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.
Codatorta was once a Knight of the Fortress of Light.