The first Castlevania game for the Gameboy, and almost inarguably the worst game in the entire series. The music is pretty damn good, just like in any Castlevania game...and that's really the only good thing anyone has to say about it.
With a protagonist that moves slower than a sedated turtle and can't jump to save his life, a lack of subweapons which are present in all the other games, a lot of sections where you MUST get invincibility to avoid getting hit as soon as you spawn, a stage that becomes unwinnable if you take the wrong path, a weapon that downgrades when you get hit, a multitude of sections where it's impossible to NOT get hit without whip upgrades, insultingly easy boss fights (except for Dracula, whose second form is actually quite tough), and tricky parkour sections that are made almost impossible by the painfully slow movement of the protagonist, Castlevania: The Adventure is often considered one of the worst games for the Gameboy.