Treasure was founded by former members of Konami, who were tired of developing sequels to the same Castlevania
games over and over. So they started a new company, and ended up creating largely obscure fan favorites for the Genesis/Megadrive that were published by Sega.
If you were to compare Gunstar Heroes
(which is a fair comparison considering its pedigree), you'd have a ton of differences. Rather than multiple lives with a single hit, you have one life with a number representing your health. Rather than the levels being the main threat, with level design itself serving as an obstacle, here the levels are mostly about the frequent boss fights, with obstacles and enemy placement being almost an afterthought. Instead of being harmed just by touching enemies, your own body can be a weapon, as you jump-attack or throw enemies. Also, you can swap and stack weapons, creating unique combos that play very differently, such as a guided flame or a thin, short, very powerful laser.
The game is rather silly. Here we get characters named after colors (Red and Blue must stop the evil Colonel Blue and his underlings, Pink, Black and Captain Orange, to save the planet from Golden Silver!), occasionally silly bosses such as a smiley face with spiked teeth, and some slapstick elements. Some of the humor is creatively used to add color to the game, such as when Black gives the player a fake gem that turns out to be a disguised bomb, or the final level, where all the previous bosses watch the player's movement on a large screen before rushing out to intercept.
What makes Gunstar Heroes
work is its frantic creativity. One level has you riding a mine cart that can switch between floor and ceiling at the player's command, before ending with a fight against a transforming robot that assumes numerous forms. Another level has players chasing a flying ship as it takes off, before they eventually get on board and ride it, ending with a boss fight set on a helicopter. One of the sillier ideas is a "dice maze", where players have to throw a die to progress along a game board, determining which challenge they'll do. You never know what you'll do next.
It's one of the most original takes on the shooter genre not only in its time, but even today you just don't see anything quite like it.