Optional Story:

• If you head back to Altair, hideout, you'll notice no one there.
• If you talk to the guy at the entrance, he'll tell you that the rebels have arisen and stage an attack to recapture Fynn.

IMPORTANT NOTE: Stock up a lot of "Gold Needle" and "Mallet" from Gatrea if you don't have "Esuna" level 6 as monsters in Castle Fynn can petrify and cast Amnesia on you.

Rebel appears near the bridge to the west of Gatrea. 
Enter the camps. 
In the camps, you can talk to people for more stories. 
Leave the camp and enter Castle Fynn to the north.

Bestiary:

• 009. Stalagmite
• 017. Dual Heads
• 018. Dual Deads
• 020. Stunner
• 025. Ghast
• 026. Revenant
• 028. Wraith
• 029. Specter
• 033. Gottos - (Boss)
• 035. Chimera
• 038. Death Flower
• 057. Changer
• 058. Brain
• 059. Parasite
• 104. Sergeant
• 105. Captain
• 107. Magician
• 108. Sorcerer
• 109. Wizard

Note: always kill Sorcerer and Wizard first as these guys can cast ailment status. 

There are many strong enemies, and it's a good place for you to level up too.

Head straight north and talk to Leila. She'll join the party. Then equip "Thief's Gloves", "Ruby Cuirass", "Ancient Sword", "Sleep Blade" or whatever weapons you like on her.

Follow the map for items:

• C-084: 2000 gil - First chest, the top-right area.
• C-085: 1000 gil - Second chest, the top-right area.
• C-086: 1000 gil - Chest in the top-left corner.

Take the bottom-left stairs (A) to 2F, then another stairs (B) to 3F. 

There are 3 chests in the top-right room (C):

• C-087: Break Tome - First chest.
• C-088: Death Tome - Second chest.
• C-089: Toad Tome - Third chest.

Optional Story: if you enter the top-left room, you'll find a giant mirror where you can interact with. An image of the Wyvern Egg appears in the mirror if you select "Key Items -> Pendant".

Backtrack to 1F (A) and take the main stairs (D) in the middle to 2F.

There are 4 chests, 2 on each side:

• C-090: Aura Tome (Monster) - Chest on the left side. 
  Monsters: Sergean x4, Sorcerer x3
• C-091: Werebuster - Chest in the top-left corner. Accessible through a hidden wall.
• C-092: Potion (Monster) - Chest on the right side. 
  Monsters: Revenant x2, Stunner, Ghast x2, Wraith x3
• C-093: Giant's Gloves - Chest in the bottom-right corner. Accessible through a hidden wall.

Take the main stairs (E) to the next floor 3F and 4F. 

Nothing in this throne room except a boss waiting for you. So make a quick save and heal up.