Snailiad (2011)

Snailiad is a classic Metroidvania/platformer indie game created by Auriplane. The game features a secret-packed retro world with multiple bosses and equipment upgrades. It also features a boss rush mode, and "Sluggy Slug" mode where you cannot retreat to your shell as a shield. The game can be found on the game's official website, Newgrounds, and a few other places like Kongregate.