Good thing is that you have already collected all treasures here except the one behind Puddingway. We'll get it later.

Now go straight to middle area talk to the well and a cutscene will trigger. A new entrance is opened to the Hollow Earth.

Walk a few steps toward south, you'll see Dwarven Castle but upon entering, you should unequip everything from Cid. 

Proceed to the throne room to fight bosses consecutively.

Bosses: Calcas and Brinas
Strategy: kill one Calca, then kill all Brina to trigger a second boss. If you kill all of them at the same time, you'll miss the second boss (again bestiary).

Boss: Calcabrina
Note: if you take too long to kill Calabirna, it will transform into 3 Calca and 3 Brina again.

Boss: Golbez
Strategy: Cecil uses Libra to check his weakness, Rydia uses summon accordingly. 
Note: Golbez can change his weakness multiple times, so make Cecil uses Libra to check whenever he changed.

After the battle, Rydia will rejoin your party, she is an adult and beautiful, isn't she? lol

Since this is a new castle, you should take time to explore and collect all the items here.

In room #9, I recommend you buy some weapons and armors to get stronger if you have some gil to spare.

In room #10, if you have already given Rainbow Pudding to the rabbit, Puddingway, he'll appear there as Mopingway. 

In the rooms #11 and #12 are special rooms, they are mostly developers. You can get Reach Augment by talking to everyone in room #11, some of them will fight you but don't worry because it won't be game over even if you lose. After that, go to #12 and talk to Fujimo, he'll ask you for a fight too. Whether you can defeat him or not, you'll still the augment.

Treasure checklist:

• 5000 gil
• Bacchus's Wine (x2)
• Hi-Potion
• Black Belt Gi
• Ether (x2)
• Elixir (x2)
• Dwarven Axe
• Power Armlet
• Silver Hourglass
• Gysahl Greens (x3)
• Cottage (x3)
• Bomb Fragment

Exit the castle.