Fork Tower and Submarine Upgrade Location and Guide
Fork Tower is one of the optional dungeons that becomes available on World 3 after you collect two of the Lithographs which allow you to unlock the Sealed Weapons (Ultimate Weapons). When you collect the second Lithograph you'll witness a scene that shows you the barrier inside Fork Tower dissipating and it actually tells you to come here. This dungeon is found just west of Crescent Town.
The dungeon of Fork Tower itself is pretty straight forward, you'll need to split your party up into two groups; one of them of Magic damage dealers and one of them Physical damage dealers. In the tower on the right you will not be able to use any Magic and in the tower on the left you won't be able to use any physical abilities. That's this dungeons unique quirk.
I'd recommend not messing around when it comes to this dungeon, I used the Freelancer class and picked the most powerful skill choices (!XAttack) on my melee characters and made sure my casters were equipped with !Summon, !White and !Time, !Red x2. You can use whatever line up of Magics you have available, I'd recommend both of them having !White Magic though and one with !Summon abilities.
The only really challenging part of these dungeons is the bosses at the end. You need to be prepared more, in my opinion, for the boss in the right tower which you can only use physical attacks against. Since you can't use curative magic throughout the fight, you'll need to take him out before you need heals or waste Elixirs on healing people.
As for the annoying caster boss in the left tower, if you accidentally melee him during the battle (which I did, stupidly, so many times on accident) he'll cast Return and start the battle over. Once both bosses are beaten, Fork Tower will collapse into the ground/ocean and a familiar place will open up.
Get in your Airship and land it on the two metal doors to find yourself back in the huge mechanical place where Cid and Mid hang out. Speak with Cid and he'll upgrade your Airship giving it submarine capabilities which are required for collecting the next tablet as well as to complete the last few side quests in the game.
Once you get the submarine back you'll be able to get the last available Jobs in FF V. People playing on the Gameboy Advance version will get more jobs than anyone playing on the SNES or PS1; instead of only being able to get Mimic (Ps1/SNES) GBA will also be able to get the Oracle, Cannoneer, Gladiator and Necromancer Jobs. These four jobs are acquired when you collect all 12 Ultimate Weapons.