Once you arrive here, talk to Gordon to learn he wants you to infiltrate Castle Fynn and to destroy the Commander. After this, exit Camp and head north and enter Castle Fynn, since Fynn itself is still overrun by Captains and stuff.
Once you enter Castle Fynn, head north to find Leila and she'll join you once more. Give her back her old Equipment, as she'll be with you for a good while now. After this, you have two options. One, you can just head due north up the stairs until you reach the room with Gottos, the Boss of this area or you can search out the Treasure Chests, while fighting off Monsters. Thing is, if you kill Gottos immediately, you can go and find the Treasure Chests right after with no Battles at all. It's your call, but to make things easier on you, I suggest just going to beat Gottos as soon as you can, so, head up the staircases until you reach him and once you do, talk to him to enter a Boss Battle. If you want a strategy, head to Section VII. Boss Guide now, otherwise, fight this yourself and move on afterwards.
After the Battle, Princess Hilda, Gordon and the Rebels retake over Fynn, so, it's safe now. After she thanks you, talk to her again and she wonders what happened to Mindu, who went to Mysidia. Memorise "Mysidia" and then ask her about it and she'll tell you what she knows, especially about the Mysidia Tower, which can only be opened by a certain way, with the uses of Masks, one white, one black. Memorise "Masks". Ask her about this and she'll tell you the White Mask is in the Castle's Depths and there's a secret password to get there, which is Ekmet Teloez. Memorise "Ekmet Teloez" and if you ask her about it, she doesn't know where it is.
So, after this, head south down the stairs and now it's time to get the Treasure Chests here. On this floor, head down the stairs again and on the next floor, head west and grab the Treasure Chest you see to get an Aura Scroll, but it's guarded by Sergeants and Sorcerers. After this, head southwest to the wall here and press up against the wall to go through it and then head north and grab the Treasure Chest here to get a Werebane Staff. Then, head east and grab the next Treasure Chest you see to get a Potion, but it's guarded by Stunners, Revenants and Wraiths. Next, from where you got the Potion, head through the wall to the east and in this new area, head south and grab the Treasure Chest here to get a Titan's Gloves.
Next up, head down the stairs in this area and on the first floor now, head southwest and go up the stairs here and on this floor, head up the stairs and on the next floor, head east and follow the path into the door. In this room, grab the 3 Treasure Chests here to get a Toad Scroll, Break Scroll and Doom Scroll. None of these are that great, nor is the Aura Scroll you got a moment ago, so, unless you really want to, don't use them, since they aren't worth it. Next, head back down to the first floor and once here, head north and grab the Treasure Chest here to get 1,000 Gil, then head east and follow the path to find two more Treasure Chests, which hold 2,000 Gil and 1,000 Gil.
After this, you're supposed to go back into Fynn, so, do so and once there, the town's lively again. When you enter Fynn, don't bother with the weapon Shop, as they don't have anything new for you, but check the armour shop out. For some reason, Gold Plates are MUCH less than Golden Mails and they're better than them… weird. So, buy 3 of them, since the person with the Knight's Armour doesn't need it and also, buy any Gold Shields you may need and Titan's Helmets as well. Next, heal up at the Inn if you need to and then head over to the house to the west and enter it. Inside is Pavel, so, talk to him and ask him about Ekmet Teloez and he'll tell you where the hidden passage is, which it's in the upper right corner of the Grand Hall, where Princess Hilda and Gordon are.
So, when you're ready, head back there and examine the wall and when asked, tell it Ekmet Teloez and a secret passage will open. In the next area, just keep going down the stairs until you reach the first floor again and in this area, before going down the stairs, try to go out of the wall here and when you do, head south and then enter the wall to the west where the Retainer person is. Once here, enter the door nearby and in this room, open the 3 Treasure Chests here to get a Gold Plate, Gold Shield and a Golden Mail. Then, head back to where those stairs where and go down them now to enter the next dungeon.