Follow the map to get inside the castle. You'll be fighting 2 bosses:

Boss: Baigan
Strategy: Kill him in this order, one arm, body, another aim.

Then walk to the throne room to fight another boss.

Boss: Cagnazzo
Strategy: Use lightning-based magic/items when there is water surrounding him, Tsunami is scary and you don't want to restart the game.
These are the lightning based that can stop his surrounding water: Thunder/Thundera/Thunder Rod (as an item)/Zeus's Wrath/Heavenly Wrath.