Tales of Phantasia - Unlockable Dungeons

Dwarven Ruins:
In the Treasure Room of the 21st level of Morlia, there is a blue button that you can press to reveal a warp for a new dungeon. Dungeon has 9 floors.

Morlia Mineshaft Lower Levels:
After you have revealed Dhaos' Castle, head to 9th floor of the Morlia Mineshaft, now there will a new opening for 12 new floors 21 in all.

Ishitori Sequence:
Whenever you play Ishitori with the Ishitori Master in Alvanista Castle, go first. Then Always bring down the amount of stones to whatever number is closest in this sequence
1-5-9-13-17-21-25-29-33-37-41-45-49-53-57...etc.
You will win every time.

Optional Summons:
There are several summons that can be obtained without going through the game, but doing certain other sidequests in the future after you have obtained the Tech Birds.

Chameleon:
Get the Emerald Pact Ring from Gnome's Cave in the future and then make it to the 16th floor of Morlia Mineshaft, he is in a room filled with gas.

Gremlin:
Get the Sapphire Ring from Undine's Cave and reach the 21st level of Morlia. He is in a room located in the south with gas.

Pluto:
Go to the Dwarven Ruins, obtain the Lapiz Ring there, and make it to the 9th floor to have a boss battle with Pluto/Brute.

The spirt of Darkness, Shadow:
Go to LoneSylph Valley and get the pact ring from the catacombs, then go to the cave of darkness to the southeast of Arlee.

The spirt of Light, Aska:
Go to Alvanista and talk to the lady standing at south entrence, then go back to the time machine in Thor and ask for Aska to be released.

Unlockables:
Battle Lv.3 Maniac Difficulty - Beat the game once.
Mini-Game - Let's Go Arche - complete the game once
New Game Mode - complete the game once
Pink Options in Title Screen - complete the game once
New Songs in Sound Mode - complete the game once Japanese version only