This achievement sorta speaks for itself... This level is the first level after you've set free Persephone (the flying elf-like thing) who allows you to upgrade your characters.
After you've set her free, before you can continue to the Stormy Stronghold, you have to visit her in the ruins. You can talk to her and back out if you don't have enough coins to spare but I highly recommend it to buy upgrades once in a while. After that you can take the balloon to the Stormy Stronghold. Once completed the achievement will unlock. Pretty easy!