The Forbidden Fortress is a palace home to the witch Maleficent and an Enchanted Forest location featured on ABC's Once Upon a Time and Once Upon a Time in Wonderland. It first appeared in the second episode of the first season of Once Upon a Time.
Before the Curse
The Forbidden Fortress is home to Maleficent and her black unicorn and rests atop of the namesake region of the Forbidden Mountain within the Enchanted Forest.
At one time, Robin Hood and his Merry Men invade the fortress while Maleficent is away in order to steal gold for poor villages. Among them, a young man named Will manages to get a treasure unbeknownst to the others. When Maleficent notices the object missing, she makes a threat upon the thieves of the consequences, but Will escapes from the Enchanted Forest.
On a mission per a deal made with Rumplestiltskin, Prince Charming fights Maleficent, in dragon form, to place the True Love Potion inside her body for safekeeping. He leaps from the window into the sea just as she aims a breath of fire his way.
Later, the Evil Queen visits Maleficent to take back the Dark Curse, which she previously traded for the Sleeping Curse. The two battle and the Evil Queen wins; taking back the scroll containing the curse.