Other than the water and frogs, it's basically your standard castle.

The women in leotards inside are a bit unusual, though.

You'd better hope you never have to find out....

Anyway, the eight priestesses are inside.

That explains the apparent obsession with nonmetal equipment, at least. There's also another interesting rumor floating around.

Sure enough, Gilbart is bedridden in one room of the castle.

Number of times someone has survived apparent certain death: 2

The doctor just confirms that he's not in good shape.

There are a few treasure rooms on the other side of the castle. The main one is blocked off by a guard, but loot what you can from the others, especially the two Ruby Rings.

Since it should be obvious what your next destination is by now, make sure you're prepared with nonmetal equipment for everyone. There's some for sale in Troy, but better options for everything but arms are available in Baron and Mysidia. Don't change equipment just yet, though.

Head north through the forest to find this chocobo-shaped area. Regular, white, and black chocobos all appear here, as well as a spot where you can use Gysahl Greens to summon the worlds hugest chocobo to store extra items. Don't leave your metal equipment behind, though—you'll want to have that handy.

This isn't a new trick, having been available nearly any time you have access to a chocobo forest, but combining healing magic with the white chocobo's ability to recover MP means free full restoration.

Once you're ready, catch a black chocobo fly to the island just to the east, and land near the cave. The airship can't land here, since there's nothing but forest, but since a black chocobo can land in only forest, everything works out just fine.

First, remember to remove everything metal anyone is wearing (affected items are noted on the weapon and armor pages. Here's the short version, based on what's actually available at this point:

Tella should be almost fine already; just swap out the Silver Bracelet he's presumably using for a Ruby Ring, and make sure he isn't using the Mithril Staff. All other rods and staves are okay. I've found that the Transformation Rod is amazingly effective, since turning things into pigs disables most special attacks, and it never seems to fail unless the target is immune. Tella should be wearing a Triangle Hat and Earth Garb, both for sale in Mysidia.

Yang should similarly be nearly ready. Keep him with the Twisted Headband and Kung Fu Suit he probably already has from Baron, put on a Ruby Ring, and you're good to go.All claws are okay, so just use whatever the target is weak against, as usual. Yang will be the mainstay of your offense here.

Cid should also wear a Twisted Headband and Kung Fu Suit with a Ruby Ring. For weaponry, he has a choice between bow and arrow or the Wooden Hammer he starts with. The hammer is less trouble and considerably more accurate, so that's my choice.

Cecil, on the other hand, doesn't have much choice but to use a bow, preferably the Greatbow, and whichever arrows fit the situation (only Iron Arrows count as metal). Stock up on Holy Arrows from Mysidia, even though they're slightly weaker than the ones sold in Troy, since many of the monsters in the cave are weak against Holy. Don't forget to put the bow in his off hand (left) for full damage, and also remember that you can switch weapons at will, which can come in handy. Alternately, Cecil can use a Healing Staff to aid recovery, though it's essentially useless for attacking. Like everyone else, he gets a Ruby Ring, but the rest of his equipment is more flexible, since he can use both the mage equipment from Mysidia and the fighter equipment from Baron. Personally, I recommend mixing the Triangle Hat with the Kung Fu Suit.

If you're playing the EasyType, this is your lucky day. The Mithril Sword in this version is replaced with the Piglet's Bamboo Sword, which sounds silly, but is actually slightly more powerful and accurate. The best part, though, is that it's nonmetal, so Cecil can fight normally! The sword isn't sold in the Mithril Town, but apparently appears in Baron's treasure room (accessable after beating Beigan).

For best results, put Yang and Cid in the front row so their attacks work normally (unless Cid is using a bow), and put Cecil and Tella in the back row (unless Cecil is using the Piglet's Bamboo Sword, if applicable). This will probably be the only time you regularly put Cecil in back.

Save the game before entering the cave, just in case you missed something. Anyone equipped with anything metal inside the cave will be stuck with an incurable paralysis-like status effect during combat. If everyone has it, that's an instant game over.

There's a save room on the third floor that shoudl come in handy. Not long after is the Faerie Claw for Yang, which has no element, but gets substantial bonus damage to Giant type monsters, as well as giving Yang a slight agility bonus and sometimes inflicting Confusion status on attack. The drawback is low accuracy, so consider not using it except when the damage bonus applies.

There's one last save room on the fourth floor, and then....

Combat begins. If he seems a bit overpowered, that's because he is. A scripted event quickly takes over, so don't waste any items.

The Dark Elf chains Fira, Thundara, and Blizzara, then hits anyone still standing with Tornado on his next turn.

Combat ends, and the party collapses.

Scene change to Gilbart's room in Troy Castle.

He struggles out of bed, only to collapse a few steps later. The doctor and nurse rush to his side.

Rather than doing something sensible like helping him walk or bringing the harp to him, they back away and watch as he struggles the rest of the way to his harp and begins playing. Scene change back to the cave.

The Dark Elf seems weakened.

The party gets up, and is conveniently fully restored.

Put all your best equipment back on, with one exception: Cecil should use the Sword of Legend, regardless of what other equipment you have, since the Dark Elf is now critically weak against Holy.

Combat goes much more smoothly this time. Cecil's Holy sword inflicts massive damage, and the triple spell combo is almost laughably weak. Tornado still hurts, but it only hits one person now, so Tella can heal easily enough. You may also see a Pig-inflicting attack, but the Transformation Rod cures that effortlessly. Anyway, cause over 3890 total damage, and after your next attack....

...the Dark Elf transforms into the Dark Dragon, and the battle arguably gets easier. All he does now is alternate between normal attacks and Dark Breath, which damages all characters by roughly 1/4 of their maximum HP. And remember that critical weakness to Holy? It's still there, so this shouldn't take long at all. On that note, you could try having Cid use Holy Arrows, but I found that the damage bonus doesn't make up for the low accuracy.

If taking advantage of a severe elemental weakness doesn't feel like enough, the Dark Dragon has an even bigger weakness: No boss flag. That means spells that normally fail on bosses can work here, though the Dark Dragon has status immunities that render most ineffective anyway. However... try casting Tornado.

There are two more odd bits of trivia here too. First, the Dark Dragon is flagged as a Dragon, which is completely pointless since the only equipment available at this point that cares is the Gold Hair Ornament, and no one in the current group can use it. More oddly, the Dark Dragon is flagged as absorbing Holy in addition to being highly weak against it. The effect of being both weak to and absorbing the same element is that physical attacks of that element strike the weakness, while magic is subject to the absorption. However, there is, again, no Holy magic available at this point anyway. It's all rather peculiar.

tl/dr version: Equip Cecil's Sword of Legend, cast Berserk on him, heal whoever gets hit with Tornado, and you'll win before you know it.

Cecil takes the Earth Crystal. The monsters should be less trouble on the way out, or you can just Teleport after leaving the crystal room.

Let's go see Gilbart first.