You can seek out the Ancient Castle as soon as you access the Falcon by entering Figaro Castle and having it move between Kohlingen and South Figaro. There's no reason to actually do this, though, so you may not realize it's there unless you fork over 100,000 GP to the thief at the Tower of Fanatics. The Treasures in the Ancient Castle are among the best in the game, but the Encounters are against fierce Monsters with high Mag.Def and Reflect, and the best items are guarded by difficult bosses. When you feel that you're ready to take on the challenge, head to Figaro Castle near Kohlingen and move it. When you bump into something, ask to stop, then head over to the prison and take the open door in the eastern cell to enter the Cave to the Ancient Castle.
Encounters in the Cave to the Ancient Castle
The Cave to the Ancient Castle consists of four interconnected areas that feature the same sets of Monsters. All of these Monsters have automatic Reflect and extremely high Mag.Def, so you'll want to focus on physical attacks. You will frequently encounter Goblins who can use normal attacks or a quadruple-damage Zap special during the course of battle. If they are the last remaining Monster, they will alternate between using one of the three numbered level Lores (L.3 Muddle, L.4 Flare, or L.5 Doom) and potentially using Blaze to deal Fire damage. They are one of the best sources for the level-based Lores in the World of Ruin. They have a weakness to Pearl-Elemental Weapons.
Along with Goblins come Figalizes. They have the most powerful basic attack in the Cave to the Ancient Castle and can Poison opponents with Gunk. They too change their strategy when no other Monsters are left alive, using either the Dischord Lores, the HP-draining Raid, or Gunk each turn.
Enuos can appear in pairs or alongside a Goblin and a Figaliz. They are similar to the other Monsters in the area in that they rely on basic attacks and specials (in their case, the Slow-inducing Slime) until they are left alone on the battlefield. At that point they can use the CleanSweep Lore as well as Aqua Rake. This is likely the first time Strago will be able to learn CleanSweep, which is essentially a higher-powered version of Aqua Rake without the added Wind Element.
Navigating the Cave to the Ancient Castle
The Cave to the Ancient Castle has a confusing layout filled with criss-crossing underpasses and hidden tunnels. The North Cavern is the simplest, but even there there is one non-obvious path to be aware of. There are three southern doorways, and the center one appears to be on an inaccessible raised plateau. However, you can pass under the wall to its west to access it directly. The chest near the western doorway contains a Wing Edge, while the one near the eastern doorway has an Ether.
The eastern room of the South Cavern contains a chest with the de facto boss of this area, Master Pug. It is hoarding the Graedus, a Pearl-Elemental dirk of great power that almost any Character can use. Defeating Master Pug is no easy task, though.
Master Pug will start off attacking normally for high damage, but approximately every 15 seconds it will use WallChange and take a step forward. After seven steps it will use Cleaver for massive, likely fatal, damage. In the meantime, it will use different Spells and Lores opposed to its current Elemental weakness, as indicated in the table. Fortunately it spends 1/3 of its turns idle instead. Finally, it has a 1 in 3 chance of countering any attack with Step Mine.
Unlike everything else in the Cave to the Ancient Castle, Master Pug has no great Mag.Def, so any attack you can offer is on the table. Just make sure that if you're trying to exploit its current Elemental weakness, the attack goes off before he uses WallChange again. The Step Mine counter will be very powerful this late in the game, s you'll want to use the strongest attacks you have so you can deal the necessary 22,000 damage in a minimal number of actions. You may want to keep a Characters with less powerful offense ready to heal the party at any time.
Once Master Pug is defeated, you'll probably want to equip the Graedus because it is especially powerful against both Goblins and Enuos. The South Cavern is even more confusing than the North Cavern, featuring triple-decker stone bridges. The least obvious path is the one through the wall west of the chest with Master Pug. You can reach the other chest, containing the Doom Darts, by either heading southwest and following the path at the bottom of the area, or taking the western doorway from the North Cavern. When you've collected the Treasures, head northwest to find the tunnel to the West Cavern.
This area is the simplest so far, but the easiest place to miss Treasures. You can proceed through a hidden tunnel southwest to a set of stairs, but there are passages to the south, west, and southwest that lead to a secondary area containing Magicite and an X-Potion. Once those are obtained, climb down the stairs to the final area, a short and Monster-free tunnel with a save point. The western stairs lead to the Ancient Castle.