During Chapter 5 you might possibly have opened up the Mi'ihen Highroad's Secret Dungeon. Getting it open is a bit tricky but once you've got it open it's the easiest of the secret dungeons.

To start out you must have started the Chocobo Ranch in the Calm Lands during previous chapters. The trick to opening the dungeon is sending at least one Level 5 Chocobo to the Mi'ihen Highroad. After sending one out check back at the ranch throughout chapter 5 until a Level 5 safely returns from the highroad. Clasko will tell you that the chocobo found a new portal on the Old Road where the hover crashed in chapter 3. Once that's done head back to the highroad and you should be able to find the portal right where Clasko said it would be.

Upon entering the dungeon you'll find a chest with some S-Bombs and a blank stone wall directly ahead. Throughout your adventures you will probably have picked up a variety of bombs. They are S-Bombs, M-Bombs, and L-Bombs - S-Bombs being the weakest and L-Bombs being the strongest. If you're running low on bombs then you should just run around fighting the fiends in this cave because almost all of them drop bombs. If you feel so inclined you can use Lady Luck's Bribe ability to get a large amount of bombs from a single enemy.

Now the breakable walls throughout the dungeon are of various strengths. The yellow walls are the weakest, green walls are in the middle, and blue walls are the strongest. To take out a yellow wall you should use 5 S-Bombs. For green walls use 5 M-Bombs. And for blue walls you'll need 9 L-Bombs.

You'll need to follow the map through the cave. If you're up for a tougher fiend you should attempt to break the false red walls - you'll be forced to fight an Epitaph. Collect the excellent items as you go through and make sure you keep fighting so you'll keep a good supply of bombs. When you get to the end of the cave you'll need to exterminate the king of the dungeon. Upon defeating him you'll receive the Font of Power Garment Grid.