Milon's Secret Castle for the NES (and now, the Wii's Virtual Console) is easily one of the worst games ever made. In this 2D action adventure, you're supposed to run around, shoot enemies, jump over stuff, and seek out items in the levels so that you can boost your character's abilities and reach later levels. Unfortunately, the game is plagued by every design gaffe you can imagine.
If you've played games like Zelda II or Wonder Boy, then you have some idea of how Milon's Secret Castle is structured. Your task is to reach the top of a castle containing four floors, each with multiple levels and a boss. Your character, Milon, can run, jump, and shoot bubbles. Shooting enemies with bubbles causes them to disappear temporarily, and they'll occasionally leave behind heart items that refill your life meter. Additionally, shooting the nondescript bricks that the levels are made of will sometimes reveal hidden passages, money, and other useful items. Shops on each floor sell magical objects that do all sorts of things, such as double Milon's firepower, let him shrink in size, and reveal invisible walls.
Milon's quest is a difficult one. Sadly, that difficulty is mainly the result of poor design choices. Items and doors are always hidden underneath blocks or are flat-out invisible until you shoot them. They're never in plain view. So, you have to waste loads of time shooting and shoving every block you see just to find what you need. While you're doing that, enemies are constantly coming toward you and hurling fireballs at you. You can shoot them, of course, but Milon's bubbles are hard to aim and they often pass right through enemies anyway. On top of that, if an enemy hits you, there's no safe refractory period in the moments afterward. Each successive touch continues to suck away your life points until you get out of there, which isn't easy because many spots seem designed to leave you surrounded. The lack of a safe "blink" period is made worse by the fact that enemies respawn within a second or two. About the only bone the programmers throw the player is the ability to continue, and this kicks in only after you manage to beat the first boss.
Without a doubt, the worst aspect is how Milon interacts with the world around him. He runs like his feet are greased, and his jumps are easily interrupted by nearby blocks that are adjacent or directly above him. For instance, in one room of the castle, you have to jump across a sequence of tiny platforms. However, if Milon's head touches the blocks situated above a platform, or if you jump in such a way that his front side touches one of those blocks before landing, you'll fall through the floor instead of landing safely. In one room, there's a gap that Milon can't fit through unless you jump just right.
Ironically, the 2D characters and backgrounds don't look all that bad, and the music is superb. As you go through the game (assuming you're some sort of masochist), you'll put together a bonus quest that involves collecting musical notes and building each layer of a song with every visit to the bonus stage. Nevertheless, solid visuals and pleasant music don't make up for a game that's otherwise so poorly designed. Don't bother wasting 500 Wii points on Milon's Secret Castle.