When you enter the Western keep you'll notice everything has been destroyed. Walk around till you find a door that you can enter. Enter it (duh), and speak with the king. The king wanted you to retrieve the crown for him. You did so. He will transform into the leader of the dark elves, Astos.
Recommended Level: 8
HP: 168
Difficulty: 3/5
EXP: 2250
Gil: 2000
The king of dark elves, Astos. He's got some nasty moves in store for your party. Reaper, which kills one party member. Slow2, which takes down their speed to practically nothing. He even casts haste on himself. If he busts out fire2/bolt2/ice2, it'll inflict massive damage upon your party. Plus, he hits a little too hard for my tastes.
If you have the Mythril sword, this battle will be easy. Use Ice 1 on him and have your fighter just pound the crap out of Astos. There's no real strategy to beating him. If he manages to kill one of your characters, restart and try again.
After you maim Astos, he'll drop the crystal eye. Return to Matoya's Cave and give it to her… not like that. When you give her the crystal eye, she gives you the Jolt Tonic. Return to Eleven Castle and give the little slave guy by the bed this tonic. The prince will awake and give you the mystic key. Recall those places where you couldn't get into earlier? Go back to these locations and pick up the powerful equipment. You'll need to pick up the NitroPowder too.
In the Western Keep, right in front of the treasure chest you'll get in fights with very powerful undead monsters. Cream them with Dia2, and gain mass amounts of EXP and gold this way.
After you're done with getting those chests, go to the northwest of the map and find a dock. Park your boat and get off. Once you're on the land, go south until you see a little cavern, enter it.