Final Fantasy II/Mission to Fynn Castle

Army camp
When you return to Altair, a man is posted by the entrance to inform you that the rebellion has been moved to a camp not far from Fynn. They are posed to strike and take back the castle from the empire. Head north from Altair, to the location just beyond the bridge which is directly south of the lake surrounding Fynn.

When you arrive, speak to Princess Hilda and Prince Gordon. Princess Hilda will express her gratitude for all that you've done and for rescuing her. And she will confirm that she is in fact the true Princess. Gordon will be very straight with you and assign you the role of infiltrating the castle as the heart of the rebellion.

Fynn
Travel across the lake and enter Castle Fynn. Shortly before you reach the first set of stairs, Leila will be waiting for you, and she will join your party.

Collect all the treasures on your way up to the top floor. Defeat the monster at the top floor and the castle will be recaptured.