does this game have any secret bosses to tackle?

[Raiden - In Flames 🎵]

slowly replaying it again since i got it for free on psn a couple of months ago.
i do remember there was a tower somewhere that was kind of difficult to go through but i think i cleared it, i don't really remember, it had a bunch of special weapons and some bosses in there. i thin it's an optional area.

[Andar]

There are three optional dungeons, all of which have various bosses to tackle.

https://grandia.fandom.com/wiki/Category:Bonus_Dungeons


Soldier's Graveyard and Castle of Dreams are both in the Zil desert, so you can clear them pretty early into the second half of the game. A lot of the equipment you get from them will last you pretty much until the final couple of dungeons.

Tower of Temptation is accessible via the Savannah Wilderness and is probably the most difficult dungeon in the game, it has a bunch of end game gear.

That said, Grandia is pretty easy and they're entirely optional.