You can visit the Centra Ruins any time after Balamb Garden becomes mobile. As soon as you enter, you'll notice a 20-minute timer that starts counting down. It will reset if you leave and return. If the timer runs out, you have the choice of trying again (starting from outside the ruins) or accepting game over. You will keep any experience or Items earned (or used) if you try again.
On a more fun note, a new entry on Selphie's Laguna page will be unlocked when you enter the Centra Ruins, much like finding a copy of Timber Maniacs.
You'll find nothing but Tonberries on the first few floors of your ascent. These are tough foes with tons of hit points. If you kill enough of them, you will fight a special boss, but hold off on that until you defeat Odin and end the timed nature of the ruins.
If you want to make things easy, use Diablos's Enc-None Ability to avoid any Encounters until you reach Odin.
At one point, you'll ride a levitating block. Immediately after, there are two ladders to choose from. The right one is a dead end with an Aero Draw point at the top.
The left ladder leads to a room with a strange device. Activate it and descend, using the now-active device at the bottom to create a spiral staircase around the tower. As you climb, you'll find a statue with one eye. Take the eye out, and continue climbing. Put the eye in the statue atop the tower to get a 5-digit code. Write this down, then take both eyes and return to the first statue.
When you put both eyes in the lower statue, you will be able to enter a code. Enter the code the upper statue gave you to open a door below. You'll find Odin inside.
When you defeat Odin, you'll earn him as a GF of sorts. He can't be junctioned, and doesn't learn Abilities, but will appear randomly before battles and defeat an enemy group. It's worth noting that you must defeat Odin before Disc 4 if you want to gain the services of Gilgamesh.
Your second task here is to find the Tonberry King. Defeat around 20 Tonberries, and after the last one dies, the Tonberry King will immediately take its place. (Note that turning Tonberries into Cards does not count as defeating them.) Defeating it earns you the Tonberry GF. This is not an easy fight, so don't hesitate to leave and save your game during the process.
On a more fun note, a new entry on Selphie's Laguna page will be unlocked when you enter the Centra Ruins, much like finding a copy of Timber Maniacs.
You'll find nothing but Tonberries on the first few floors of your ascent. These are tough foes with tons of hit points. If you kill enough of them, you will fight a special boss, but hold off on that until you defeat Odin and end the timed nature of the ruins.
If you want to make things easy, use Diablos's Enc-None Ability to avoid any Encounters until you reach Odin.
At one point, you'll ride a levitating block. Immediately after, there are two ladders to choose from. The right one is a dead end with an Aero Draw point at the top.
The left ladder leads to a room with a strange device. Activate it and descend, using the now-active device at the bottom to create a spiral staircase around the tower. As you climb, you'll find a statue with one eye. Take the eye out, and continue climbing. Put the eye in the statue atop the tower to get a 5-digit code. Write this down, then take both eyes and return to the first statue.
When you put both eyes in the lower statue, you will be able to enter a code. Enter the code the upper statue gave you to open a door below. You'll find Odin inside.
When you defeat Odin, you'll earn him as a GF of sorts. He can't be junctioned, and doesn't learn Abilities, but will appear randomly before battles and defeat an enemy group. It's worth noting that you must defeat Odin before Disc 4 if you want to gain the services of Gilgamesh.
Your second task here is to find the Tonberry King. Defeat around 20 Tonberries, and after the last one dies, the Tonberry King will immediately take its place. (Note that turning Tonberries into Cards does not count as defeating them.) Defeating it earns you the Tonberry GF. This is not an easy fight, so don't hesitate to leave and save your game during the process.