A guard is waiting near the entrance to Altair, instructing you that the rebels have set up a Camp to the northwest, near Gatrea. You can still shop in town or stay at the inn, but you'll need to make the trek to the Rebel Camp to speak to Hilda, Gordon, and the other NPCs of the rebellion.
Rebel Camp
This small staging area was made in preparation for an assault on Castle Fynn. Hilda asks you to lead the charge in liberating your home, and that should be your next priority. The Rebel Camp will only be present for a short time, but there is nothing of interest to find here aside from plot progression. You can reach Castle Fynn easily by crossing the lake to the north.
When heading to the secret basement, there is a gap in the east wall that leads outside the castle, and another gap to the south leading back into the castle where you will find a Treasure room.
Castle Fynn 1F
You'll find Leila waiting for you on the 1F of the castle. She'll rejoin with the same stats and gear she had when she left the party after the Lamia Queen fight. You now have a choice to make. Castle Fynn contains lots of nice Treasure, which you can go hunting for right now. Alternately, you can proceed up the stairs to the 2F and just keep climbing stairs until you reach the boss, Gottos, in the throne room. Once defeated, all of the Monsters in Castle Fynn will disappear, with the exception of the ones guarding specific chests.
The Monsters here are similar to those found in the Coliseum, so I would recommend going directly for Gottos rather than risk being annihilated by Wizards in some random encounter while exploring the Castle. Nonetheless, I will describe the Castle's layout and encounters in case you decide to hunt for Treasure first.
As mentioned, most of the encounters in Castle Fynn were previously seen in the Coliseum, and on the 1F there isn't a single new encounter to be found. You won't find any Parasite, but you will find the Stalagmite group that was guarding an Antidote as a Random Encounter. Most importantly, the Captain/Sergeant/Sorcerer/Wizard encounter (hereafter just referred to as the "Wizard encounter" because let's be honest about the threat here) is just as common as it was on Coliseum B1, so don't waste time. Keep in mind that the inn in Gatrea is just a Teleport Spell and a short walk away if things go poorly in any given battle.
There are three Treasure chests and one side stairway to find on 1F. The hall leading east from the main entryway leads to a pair of chests with a total of 3,000 gil. The hall leading west splits, with another 1,000 gil at the end of the first northern branch, and stairs up to the 2F at the end of the southern branch. This path leads to a trio of powerful new Spell tomes on the 3F.
Castle Fynn 2F
The Monsters on the 2F are largely the same as the ones on the 1F. Magicians are gone, replaced by a rare encounter featuring both Parasites and Brains. Brains like to use Swap, which will likely decrease the target's HP and restore their MP somewhat. They can also cast Fog, which is a problem if it inflicts Amnesia on your Esuna caster. Esuna 3 (outside of combat) will remove Amnesia, as will a Mallet. More urgently, the Wizard encounter is now twice as common as it was on the 1F.
Surprisingly, the side route to 2F leads immediately up to 3F. The bulk of 2F is found along the main path to Gottos. Once again, you'll be faced with paths diverging to both east and west. Each leads to an easily reached chest, as well as a second chest hidden behind a false wall.
To the east, you'll find a mere Potion guarded by an eclectic combination of Revenants, Ghasts, Stunners, and Wraiths. The false wall is nearby, through the northern half of the nearby dead end. You'll find a pair of Giant's Gloves if you head south from there. To the west, you'll find another guarded chest, albeit with a better prize: an Aura Tome. It's guarded by Sergeants and Sorcerers. Sadly, Aura is pretty much outclassed by Berserk, despite being interesting in concept. Head to the southernmost dead end and you can pass through the northern half of its western wall. You'll find a Werebuster at the north end of the hall.
Castle Fynn 3F
There are fewer Specters on the 3F, and the Parasite/Brain combo is more common. Mercifully, the Wizard encounter is no more likely than it was on the 2F. There is a new rare encounter with as many as eight(!) Sorcerers. Even with their relatively low HP, that can get ugly fast.
The path to Gottos is straightforward and short, leading almost immediately to the throne room. Approach the NPC on the right throne and speak to him to trigger the boss fight.
The side path from 1F emerges here in a U-shaped hallway with two rooms at its endpoints. The western room contains a mirror that will be useful later, but has nothing of value for you yet. The eastern room is a Treasure Room with three powerful KO Spells: Death, Break, and Toad. Toad is objectively the best of the three, and there's no practical reason to learn either of the other two. However, in addition to Treasure, this Treasure Room has its own encounter list. It's more of the same groups you've seen, but the Wizard encounter has an almost 1/5 chance of appearing. Don't spend any more time in here than you need to!
Gottos
Gottos's only form of offense is attacking, but he will sometimes use Imbibe 6 or Haste 6 to make his attack that much more powerful. You'll want to take him down as quickly as you can before he builds up too much power, though his 2,000 HP, 60 defense, and lack of Elemental weaknesses make that difficult. The usual offense of Berserk and Curse is probably your best bet. Blink is, as ever, a good defense as well. Gottos gets a ridiculous six attacks, so you'll need every bit of defense you can muster. His magic resistance is variable enough that status ailments have a decent chance of connecting, though.