The North Palace is a blue pyramid-like structure from Chrono Trigger. The Kingdom of Zeal built it on an island in the sky during the Dark Ages. The Zealites put dangerous elemental weapons in the palace. Queen Zeal locked it after she built the Mammon Machine. Crono and his friends can't access it during the Dark Ages since they can't access the island.
After the fall of Zeal, the North Palace fell to the bottom of the ocean. It resurfaced around 600 AD in the middle of a forest, however, Crono and his friends still can't access it because the forest is too dense. A path through the forest is cut by 1,000 AD and Crono and his friends can access it. It is referred to as the Forest Ruins at this time.