As you enter this place, head southeast and grab the Treasure Chest here to get a Knife, then head northwest and grab the Treasure Chest here to get an Eye Drops. Next, head east and then go south when you can to find another Treasure Chest which holds a Shirt, then go northeast and go down the stairs here. On this floor, head east to find a Treasure Chest, which holds a Potion, then head west and grab the next Treasure Chest here to get another Potion. Next, head north and grab the Treasure Chest here to get a Maiden's Kiss, then head east and head down the stairs you come across. On this floor, head southwest and grab the Treasure Chest here to get a Flame Shield. Give this to Firion if he can hold a Shield, otherwise, give it to anyone else.
Next up, head east and grab the Treasure Chest you see to get a Potion, then head northwest and open the Treasure Chest here to get a Silk Web, then just head south and go down the stairs. On this floor, follow the path west and when you can, go to the east to find a Treasure Chest that holds a Blood Sword, which is guarded by Ghasts, Revenants, Specters and Wraiths. After this, make SURE you hold onto this, as this Sword is the key to the final Boss in the game. SAVE IT AT ALL COSTS. Don't bother using it now, since it's rather useless and instead, just head south now and go northeast when you can to find another Treasure Chest, which holds a Flame Mail.
Give this to Firion and then give the Knight's Armour to who ever if you had it on him. After this, head southwest and follow the path south and grab the Treasure Chest you see to get a Backstabber, then head south down the stairs. On this floor, head east as far as you can, then north and grab the Treasure Chest here to get an Orichaldagger, which is guarded by a Rhyos. Next, head northeast to find seven doors and enter the door that's second to the right and in this room, head north and open the Treasure Chest to get the White Mask! Afterwards, head north and step on the teleporter here to exit this place.
Outside now, go and see Princess Hilda if you want, but all she tells you is to take the White Mask to Mysidia. Anyway, make sure you sell off any Items and stock up on whatever you need, PLUS make sure you DEFINITELY heal up at the Inn and when you're ready, look at your Map. From Altair, to the southwest, you see that pink square? That's where Mysidia is. You'll have to use your Ship and go around down to the south and then enter the small sea like area to the west of it and land there, so you don't have to walk far. Once there, though, watch out, as there's some INSANELY strong Monsters here. I STRONGLY suggest at this time, you train again and you should do it, as if you do, you shouldn't need to train anymore for the rest of the game.
I suggest you get at least 2,000 HP with everyone, about 300-400 MP with your Magic users and at least Level 10+ spells with whatever you want. weapons/Shield levels should at least be 10+ as well. I know this sounds like a lot, but trust me, you WILL want to do this to save you time and frustration later on. If you don't want to do that much, at least get 1,500 HP, 250 MP and Level 8+ spells or so, with weapons/Shield at about 8+. That should be reasonable at least. Anyway, before you start, go ahead and enter Mysidia.