Not a lot to say. Continue along to the room past the entrance room. There, speak with a guard by the castle and you'll watch some dramatic scenes, revealing the truth behind the king's insanity.
The boss's attack repertoire is fairly small. There is a basic physical that barely more powerful than other enemies' at this point, and a Lightning/Thunder attack that hits the whole party.
Remember all the Dragoon stuff you got from the rest of Saronia? You should have plenty to supply four Dragoons, and grinding Dragon Spire should have gotten you up to Job Level 10~15. Using Jump at that point should be able to deal about triple the normal amount of damage, and throwing in Wind Spears really can result in one Jump dealing 2,500~3,000 damage with ease. If you feel like it, you can even bring along one Bard with the Loki Harp to boost your offensive power by 20%~25% for one member, and it stacks if you perform it again and again!
And if your Dragoon Job Level is sitting at one or two? Well, you should still be able to deal decent damage... But you can always bring along other jobs, I guess. Or go grind before coming here to fight! The Aero spell does great here, and the Dark Knight with the Blood and Royal Swords has no problem dealing around 800 damage at low Job Levels with Souleater.
After the post-fight scenes, let's loot the castle!
Go through the east-central wall and along the long path to find a dozen treasure chests. Take your time in opening them for an Ice Rod , a Tome of Fire , a Tome of Ice , a Tome of Light , a Golem Staff , a Medusa Arrow , a Rusty Mail , a Main Gauche , a Scholar Hat , a Selene Bow , a Boomerang , and a Dragon Hammer .
Next, go down the stairs nearby. In the next area, go south into a large room, then go along the hidden path on the east side of the room to find another six chests. These contain another Rusty Mail , a Viking Helm , some Viking Mail , a Dragon Helm , a Gaia Vest , and an Earthen Bell .
There are two buildings nearby: one to the west and one to the east. Go into the former first. Go into the hidden passage to the east and go to the next area. There, go west, through the wall, north, east, south, and west to the next area.
Continue along the linear path to find two Phoenix Downs . Backtrack out of here.
Back in the castle courtyard, go into the eastern building. Speak with the four guys for a ship upgrade. Sweet! Leave the castle so we can destroy it again! Just kidding. The Nautilus goes crazy fast through the sky, so no more slow traveling for now.
Let's also consider visiting the various shops that have been unlocked in the four Saronian towns.