Liberated Castle Fynn
As soon as Gottos is taken out, the rebels move in to Castle Fynn. Hilda and Gordon take their places on the two thrones. Hilda will mention that Minwu went to [Mysidia]. Asking her about [Mysidia] leads to a discussion of [Masks]. She indicates you need two to break a magic barrier and enter the Mysidian Tower. The good news is the White Mask is hidden in the Castle Fynn Basement, and she even knows the chant that will open the door: [Ekmet Teloess]. She doesn't know where the door is located, though. You'll need to find someone who does know.
You can get some information from Gordon as well. Ask him about [Mysidia] and he will mention that the [Ultima Tome] is hidden in the Mysidian Tower. If you ask him about [Ekmet Teloess], he suggests that Paul may know more about the castle.
Before moving on to Fynn, feel free to collect any treasures you skipped before taking down Gottos in Castle Fynn. The Potion and Aura Tome are still guarded, but otherwise you can explore at your leisure.
Fynn
With Castle Fynn retaken, the imperial guards and Monsters in Fynn disappear as well, and the town's Shops open up. This is one of the last towns in the game where you can buy new stuff, and by now you probably have a ton of gil. The Weapons for sale here are all things you've seen before and probably don't need, though the prices are quite reasonable. The Armor, on the other hand, might be useful. The Gold Cuirass is the best Armor money can buy, with the same Defense as the Knight's Armor but 21% less of an Evasion penalty, all for the bargain price of 1,000 gil. You will very shortly find another Gold Cuirass (as well as Golden Armor and a Golden Shield) for free back in Castle Fynn, too. Don't forget to upgrade Leila's gear, as she'll be sticking around for a while.
The Spells here are also nice, the first new white magic you've seen in a while. Dispel is bugged and is useless, but the other selections are solid. Mini is a fairly accurate white magic KO Spell, while Fog is a version of Silence that won't wear off. Whether that advantage is worth its 10% lower Accuracy will determine which Spell you favor. Slow is the real prize, though, as it help reduce the number of attacks Monsters make, which is a great boon in the late game.
Paul's house is in the southwest, and he can teach you where to use the [Ekmet Teloess] password in Castle Fynn: in the top right corner of the throne room. Make sure to visit Paul as events proceed in the future, as he has several nice rewards for you.