Gold Tower, in reality the Light Palace, is a location in Secret of Mana. Located on the Isle of Gold adjacent to Gold City, it can be entered as soon as Flammie is acquired. As a Mana palace, it is the sixth to be visited when following the story.
Lumina, the Light elemental, was captured and forced into servitude by King Mammon, who commanded that the Isle of Light be transformed into an island of pure gold, thus creating Gold City and the Gold Tower from which he ruled. As he created the most lavish city in the world, Lumina suffered in torment, granting his wish.
As the power of Mana continues to fade, Randi and co. are sent on an errand by Sage Joch's disciple, Jehk to search for him. From the Palace of Darkness, they are told to fly to Gold Isle if they wish to find Joch.
Mammon sealed the Gold Tower with a key, passed on elsewhere in the Empire and placed under guard. Mara, a Tasnican agent in Southtown, holds the key in lieu of her husband, who was caught and killed while retrieving said key.
The party makes its way through the tower, key in hand, and destroys Mammon's ancient guardians, setting Lumina free to serve the white mage Primm as a token of her thanks.
The Light Palace can only be entered once the Gold Isle Key has been obtained. In the city, if go up the wall, loop around the whole city thing and talk to the guy at the end, you will learn that the key has been stolen by a Tasnican spy from Southtown. Mara was a Tasnican, and a spy, AND from Southtown! Board Flammie and visit her. Enter Mara's house and talk to her. She will say that her husband was caught stealing a secret key on Gold Isle and she finds it and will give it to you.
In the first room, beat the two Beast Zombies, and take the Spear's Orb out of the chest. Return to Gold City to forge it, and return. Go to the next room. Fight the Dark Ninja and the Wizard Eye, go through the door and up the stairs, and it is immediately boss time (Blue Spike).
Once you defeat Blue Spike, a stairway pops up. Take it, and you will arrive in a familiar room. Beat the enemies and circle around, up the stairway, to meet Gorgon Bull.
A stairway similar to the one that appeared after defeating Blue Spike emerges from the floor. Go up. Talk to Lumina, the elemental in front of the pedestal, and the girl will gain her power! Seal the seed's powers and use the Magic Rope to get out of here.