You can come here and collect a ton of free loot as soon as you get your first airship, but plotwise there's no reason to come here until after you've been to the Underworld. While there's plenty of unguarded Treasure, three chests have encounters that will be quite nasty if you come here early. Items like an Hourglass1 will certainly help, though. Rydia is useful in these fights, as Ifrit can do a number on the Skulls guarding the Sleep Sword and the Mist Dragon is an effective way to break through the Mad Ogres' considerable Magic Defense.
Actually finding every chest can be tricky. In the middle area, the eastern chest can be found by walking right through the wall. You can find stairs to the 2F chests from the Throne Room a level up by walking through either side of the wall at the back of the room. Get used to walking into walls to find chests that are not accessible from the central floor.
In the west tower, walk through the east wall one square above the door to find the guarded chest with the Sleep Sword. Two pots on the second floor each hold a Bacchus, including the one in the southeast corner that has no obvious access point (walk through the southern wall between two pots). That same path leads to the two chests. The conspicuous dead end on the west side hides stairs to the Kamikaze on the first floor.
The Coffin in the east tower is accessed by walking through the wall above the northwest column. Most of the Treasures on the third floor are fairly obvious, though you need to walk through the southern wall to get to the door from the switch that opens it. Walk through the northeast wall to access the guarded chest behind the black pit square. All of the Monsters in this fight have nasty counterattacks, so try to take them out hard with a Spell like Titan. You can then fall into the pit to get the second floor chest, and find stairs to the basement. Watch out, the lone chest on the east side of the path is guarded by Mad Ogres.
Actually finding every chest can be tricky. In the middle area, the eastern chest can be found by walking right through the wall. You can find stairs to the 2F chests from the Throne Room a level up by walking through either side of the wall at the back of the room. Get used to walking into walls to find chests that are not accessible from the central floor.
In the west tower, walk through the east wall one square above the door to find the guarded chest with the Sleep Sword. Two pots on the second floor each hold a Bacchus, including the one in the southeast corner that has no obvious access point (walk through the southern wall between two pots). That same path leads to the two chests. The conspicuous dead end on the west side hides stairs to the Kamikaze on the first floor.
The Coffin in the east tower is accessed by walking through the wall above the northwest column. Most of the Treasures on the third floor are fairly obvious, though you need to walk through the southern wall to get to the door from the switch that opens it. Walk through the northeast wall to access the guarded chest behind the black pit square. All of the Monsters in this fight have nasty counterattacks, so try to take them out hard with a Spell like Titan. You can then fall into the pit to get the second floor chest, and find stairs to the basement. Watch out, the lone chest on the east side of the path is guarded by Mad Ogres.