You can enter the Unidentified Castle through a massive warp portal located directly atop the hill.
It's called the Celetial Castle of Rahkon now, but this fact was discovered by Norberto by studing ancient texts and relics, and it was also Norberto who erected the warp portal, the exclusive gateway to the Celestial Castle Rahkon.
But that's the extent of the discovery, nothing more. A handful of mercenaries have been conducting lengthy investigations but their efforts have been hampered by monsters dwelling inside the castle.
Accordingly you must offer Norberto aid whereupon he will welcome you profusely and retain you for numerous tasks. Solve the myraid secrets ensconced in the Celestial Castle Rahkon according to his instructions.
As Norberto can be found in Rahkon's first floor, enter the castle via the warp portal on the hill, as I stated earlier.