FF3: What's with this unreachable boss in Salonia Castle?

[Peter_20]

Hey, I was walking in this secret passageway in the Wizards' room, and suddenly I spot some boss-ish guy standing on the other side of a wall, like "come here and fight!" or something.
How do I reach him?
There's this guy saying something that the room used to lead to the catacombs, but that wasn't too much help.

[Sefie1999AD]

You need to take your submarine Nautilus underwater and then use it to enter a cave near the area where Salonia is. You'll enter Salonia catacombs, and you'll get to fight the boss at the end of the dungeon. However, be prepared for a very tough boss fight. Hint: Use Fire spells. Or Dragoons with Jump.

[Kawaii Ryûkishi]

That guy certainly has a thing for hanging out in hard-to-access castle basements.