Hydra Castle Labyrinth, originally Meikyuujou Hydra, is a freeware Indie platformer developed by E. Hashimoto, a.k.a. Buster, and released on his website. The original game was entirely in Japanese, but saw a translation patch months after its original creation.
The game hearkens back to early "Metroidvania" games like Knightmare II: The Maze of Galious and Legacy of the Wizard: the player explores an immense maze-like dungeon, collecting tools and keys that allow them to proceed deeper. Certain areas are cordoned off from the main dungeon area, and each presents a more linear zone with a boss at the end. The player can find new equipment to make themselves stronger as well.