This does not end well
Kid Icarus is best described as a mix between a side-scrolling shooter and a platformer, incorporating elements of both. The angel Pit, your character, will have to jump from ledge to ledge and use his magic bow to fight off enemies from Hell itself. While the premise isn't all that original, it's decent, and nothing to hold against the game.
The rest of Kid Icarus is a completely different story. A sadistic difficulty level, infinitely spawning enemies, and low health are the most blatant offenders, although far from the only culprits. As Pit ascends from platform to platform he must fight off his enemies with his bow. here is where the problems begin: missing a platform and falling to the bottom of the screen means instant death. It's impossible to backtrack and pick up an item or enter a door you may have missed, and enemies will spawn infinitely until you advance past their spawn point.
Fighting off enemies is a simple matter of shooting them with your bow, but even that is tougher than it sounds. Pit's arrows don't travel very far, meaning to kill an enemy you must be practically touching it, which, for obvious reasons, doesn't end well most of the time. When killed, enemies leave behind hearts which can be used to purchase upgrades and items in the game's rare shops. Some of these upgrades (Such as additional health) serve to make the game easier, but they are so difficult to obtain, it's a moot point. The rest of the game is as afflicted as the basic gameplay. Small, bland, barely animated sprites that weren't all that impressive even back during the time of the NES comprise the graphics, which hold up horribly. A single-color background and some platforms to jump between is all the scenery you'll be taking in on your journey, and the chirping, repetitive music that accompanies the bland sprites certainly doesn't help.
Overall, Kid Icarus is a sadistically difficult game, that doesn't deserve it's reputation as a classic. Underwhelming graphics, even at the time of it's release, poor sound, and bad gameplay combine to form Kid Icarus. As if adding insult to injury, Nintendo has removed several of the cheat codes from the original game, and hasn't even bothered to remove the slowdown that occurs when multiple enemies crowd the screen. Kid Icarus was a mediocre game when it released in 1987, and it's no better now, 20 years later.