Note: The Generals here have some REALLY good Item Drops. If you get them, USE THEM. They drop the following that's useful: Diamond Gaunlet, Diamond Helmet, Dragon Armour and Rune Axe.
Alright, as you enter this place, head north and enter the door you see and in this room, just head north until you fall down to the first floor. Once here, head south and get the Treasure Chest you see to get a Diamond Plate and give this to whoever needs it the most. After this, head back north and go east as far as you can, then head south and head up the stairs you see. On this floor, just follow the path and open the Treasure Chest you see to get a Hourglass, then head back down the stairs. Next, continue south and when you can, head as far west as possible to find a Treasure Chest, which holds a Garlic and after this, backtrack east and head north and go up the stairs here.
On this floor, follow the path and head south when you can to find a Treasure Chest that holds a Knife. Then, head north and open the Treasure Chest here to get a Buckler Shield and then head west and follow the path and go up the stairs you come to. On the next floor, head north as far as you can and then head west and follow the path to find a Treasure Chest, which holds a Thunder Spear and is guarded by Fallen Emperor! Apparently, he's trying to evoke his revenge on you here, but he's a pushover. After the Battle, give this to anyone who uses Spears, as it's much better than what they have now, then head back north and head east and follow the path around and go up the stairs here. On this floor, follow the path and go southwest when you can to find a Treasure Chest that holds a Saint's Spirit, then head west and get the Treasure Chest here for a Slumber Stalk.
Next up, head northeast and when you can, head west down the narrow path and follow it down to the south to find a door. Enter it and inside this room, open the 3 Treasure Chests here to get a Spellbinder Staff, Mage's Staff and a Healing Staff. Don't bother with the Healing Staff, as when it hits a target, it heals them. This is good for healing your party members and against the undead, but any other time, forget it. Anyway, exit the room and follow the path all the way around and head east and go up the stairs here. On the next floor, head west and go up the stairs here and on this floor, head south when you can to find a Treasure Chest that holds a Sunblade, but it's guarded by Lamia Queens and Coeurls. After the Battle, give this to Firion or whoever else uses Swords, but don't bother with Gareth, as he'll be leaving you shortly.
Afterwards, head east and follow the path up to the stairs here and on the next floor, follow the path here and head through the door to the west. Inside, grab the 3 Treasure Chests here to get 3 Elixirs! Damn, nice indeed. Next up, head back down 2 floors and back on the 5th floor now, take the south path here and head up the stairs you come to and on the next floor, just follow the path and head up the stairs to the next area. On the next floor, just follow the path and head up the stairs and on this floor, follow the path and head west when you can and follow the path here to find 2 Treasure Chests, which hold a Hellfire and a Zephyr's Flute. After this, head into the middle of this room to find Leon and before you talk to him, remove all of Gareth's Equipment, then proceed.
As you speak to him, he challenges Firion, but Maria stops him, though, he won't listen, as Emperor appears, "reborn". He laughs at Leon thinking he was the new Emperor, but Leon thinks he's just a ghost, as blue flames circle around your party. Emperor announces he no longer cares about anything and will destroy the world, as Gareth decides to hold him off while the others escape on the Wind Drake. Afterwards, the place apparently collapses, as you arrive back in Castle Fynn, as Princess Hilda asks what happened. Firion fills her in, as Leon willingly joins your party to put an end to all this and once you regain control of Firion, check Leon out. You now have your permanent 4th party member, as you'll NEVER lose Leon again. For his Equipment, Leon has stats in every weapon, but I suggest you train him either with a Sword or Axe.
It's your call, but either will do nicely. Just remember he's Left-Handed, so fit his weapon in his Left Hand. As for Equipment, don't bother with anything except the weapon/Shield, as his stuff is top notch. HP is very good and he has only 5 MP, but he can start gaining a TON of you train him. Though, before you do train, there's one place left to go. So, get into your Airship which is outside of Fynn and head to Castle Deist and once there, enter it.