I've discovered an unused level (level 16) coming after "The Goblin Fort" but before "Joy Castle", called "Ruins of Joy Town".
Here's a video of it.
As far as we're aware at the time of writing this, there is only one PC version, so this guide should work for everyone. If it does not, post here so we can look into it.
1) With a hex editor, open C:\Program Files (x86)\Konami\Frogger - The Great Quest\data.bin. (Run the hex editor as admin, THIS IS IMPORTANT) 2) Replace 4 bytes, starting at 0x278 with 1A F2 08 72. 3) Replace 4 bytes, starting at 0x28C with 2A F2 22 72. 4) Save.
You're done. Now when you load "The Goblin Fort" in-game, it will load the unused level instead.