EXDEATH'S CASTLE
My Level: 40 (Bartz), 39 (everyone else)
Before heading inside make sure you have the ability from the Geomancer class that allows you to see where there are traps on the floor. Equip it on Faris. You'll also need Learning and Control for some new Blue spells if you want to learn those.
In the castle you find the soldiers had been defeated by Exdeath. Go upstairs. The two chests in here have a Diamond Shield and an Ether potion. The next floor has an illusion that fools everyone but Krile into thinking it is a dead end. Kelger, the last remaining ally of Galuf's uses the last of his strength to undo the illusion. He passes away. All the original warriors are gone. You can get up into Exdeath's tower now though and you can see the castle in its creepy true form.
The next floor after this is fairly straightforward. Move along the path until you see a switch. Flip it. To the right of the switch is a chest with an Ice Shield that you can now open since the switch opened up the wall blocking your way earlier. Proceed south and follow the path into the next room, which is also straightforward. There is a chest with an Ether in here, keep following the path. Now go into the next room. In this room you can go down into the lava but it will damage you, so don't. Walk around on the upper path that is above the lava. There is a chest north of the door that you can open for an Elixir. Don't miss the chest east of the door too, you can reach it through an invisible path in the wall. You need the Geomancer's Light Step ability to make it there safely since you do have to go down into the lava to get to the path. This chest has a Hayate Bow. Now go back the way you came and go upstairs.
In here is a puzzle with a skull switch. Step on the skull to make the platform move, step off to make the platform stop. You can use this to reach the two chests and then the stairs. Or just the stairs. If you open the chests you will get an Icebrand Sword and a Kotetsu. SAVE in the next room! Now go upstairs. In here you will see a chest hidded within the walls. Go east toward the chest. The invisible path leading to it is not paralel to it but two spots north of the end of the hall, now go right and down to get to the chest. It has an Elixir. Now make your way over to the west and grab the chest in the left corner by the door. It has 9900 gil. Go upstairs.
CARBUNCLE SIDEQUEST
My Level: 40 (Bartz), 39 (everyone else)
This gets its own section since this is the ONLY time in the entire game that you can do it. Make sure you do it RIGHT NOW if you want Carbuncle as a summon. You'll see a green summon orb similar to Shiva back in world one. That's Carbuncle, but you cannot get to him. Yet. Go down the stairway to your right. Here you'll want to put your Light Step ability to use again since you're going into the lava. There are two ladders to the right of you. Take the one furthest from you first. It leads to a chest with 8000 gil. Head back down into the lava again and take the ladder left of where you just went down. Notice the skull tile and ladder in the middle. That leads to Carbuncle. Once upstairs again switch out Light Step for Find Pits. There are holes in the floor here. Make your way to the chest east of you for a Twin Lance. Now go to the skull at the northern point of the puzzle. It warps you to Carbuncle. Again, just to make it clear, this is the only time during the game when he can be battled for a summon pact so do it now or else you will miss out on a summon spell.