This game is amazing! And not just amazing for the NES, but amazing in general, lots of fun! It's really built with multiplayer in mind, so try to find some friends and experience the multiplayer mechanics, like bouncing off your fellow mages or being able to float around as a ghost looking for ways to come back to life.
But it's also fun to play on your own! The game isn't very long. A dedicated player can probably finish it in about two hours. It includes a hard mode, which you need to complete in order to see the true ending, kind of like classic NES games like Ghosts N' Goblins. The difference is that the second loop in hard mode has lots of little changes, from level layouts to enemy behaviour, that really are more like just the second half of the game, kind of like the inverted castle in Symphony of the Night. There are also secrets to uncover and lots of little easter eggs and small references to other games, as classic as Space Invaders and as recent as Shovel Knight.
If you like run-and-gun wall-jumping vertical action, definitely give this game a try!