The Melc Ruins are a floating set of ruins located in high in the skies above the ocean near the Vejas Jungle. Yuki and the rest of the party go there to meet with Unama, one of the Guardians. The depths of the ruins are guarded by the infamous Melc Crystals.
Despite its name, the Melc Ruins are completely and fully operational with little to no actual ruins in the place. It's also known more appropriately as the Sky Castle.
The Melc Ruins are the first place the player can fight Lucky Minks which, like Dragon's Quest's Metal Slimes, are difficult-to-defeat enemies that yield vast amouns of experience despite their low HP counts.