Talk to Hilda, then select:

• "Learn -> Mysidia"
• "Ask -> Mysidia"
• "Learns -> Mask"
• "Ask -> Mask"
• "Learns -> Ekmet Teloess"

Next, talk to Gordon, then choose:

• "Ask -> Mysidia"
• "Learns -> Ultima Tome"

There is a whole new dungeon beneath the castle. To unlock the basement, Go to the top-right corner in this room, then examine the wall (A) and select "Ask -> Ekmet Teloess". 

Keep going down from 4F (B) -> 3F (C) -> 2F (D) -> 1F (E). 

Between the narrow corridors, there is a secret passage at the right wall providing access to the room below.

• C-094: Golden Armor - First chest.
• C-095: Golden Shield - Middle chest.
• C-096: Gold Cuirass - Right chest.

Backtrack to the corridor and take the stairs (F) to B1.

Bestiary:

• 009. Stalagmite
• 012. Adamantoise
• 013. Land Turtle
• 015. Ogre Mage
• 016. Ogre Chief
• 020. Stunner
• 025. Ghast
• 026. Revenant
• 028. Wraith
• 029. Specter
• 035. Chimera
• 036. Rhyos - (B5: Chest Monster)
• 057. Changer
• 058. Brain
• 059. Parasite
• 081. Green Soul
• 087. Killer Mantis
• 107. Magician
• 108. Sorcerer
• 109. Wizard

• C-097: Knife - Chest in the bottom-right corner.
• C-098: Eye Drops - Chest in the top-left corner.
• C-099: Clothes - Split path near the exit.

Take the exit (G) to B2.

• C-100: Ether - Top-right corner near the entrance.
• C-101: Potion - Bottom-left corner.
• C-102: Maiden's Kiss - Top-left corner.

Head to exit (H) to B3.

• C-103: Hi-Potion - Bottom-right corner.
• C-104: Flame Shield - Bottom-left corner.
• C-105: Spider Silk - Top-left corner.

Take the stairs (I) to B4.

• C-106: Power Staff (Monster) - Top chest. 
  Monsters: Ghast x2, Revevnant, Specter x2, Wraith x3
• C-107: Flame Armor - Top-right chest.
• C-108: Betrayal Fang - Bottom-right chest.

Take the exit (J) to B5.

There is only one chest in the middle. 

IMPORTANT: Make a quick save before opening the chest. These monsters can petrify your entire party.

• C-109: Orichalcum (Monster) 
  Monsters: Rhyos x4

Next, take the 5th door (K) from the left. Others are just empty rooms.

• C-110: White Mask

Step on the portal (L) at the top to exit.

• The town is now free from the empire. All new shops are also available.
• Use INN to fully restore your HP/MP.
• The only item I recommend you buy is the "Gold Cuirass" for everyone. This is an upgrade from "Ruby Cuirass".

Optional Story: Paul's house is in the southwest of the town. If you talk to him and choose "Ask -> Ekmet Teloess", he'll give you a hint on where the secret passage to the Castle Fynn Basement which we've just finished.