Its like one of the most advanced and funnest NES games ever made, I'm not entirely sure how you could have missed it. I'd think its almost as well known as some of the more mainstram games like mario and zelda and megaman.
Wiki link:http://en.wikipedia.org/wiki/The_Guardian_LegendYoutube reviewhttp://www.youtube.com/watch?v=mng_DmrWdoYBoxart:
Gameplay:


Basically its like one of the best NES games which I guess isn't all that big of a deal anymore but basically it combines zelda1 type of exploration and shmups. You start off in a shmup stage, nothing too much to mention here, then you get to the overworld from where you can explore, fight mini-bosses, buy new weapons, level up etc.
Your goal is to seek out all the dungeon gates and figure a way to open them up from various hints and trial and error basically. The dungeons are the shmup stages and once you clear them you get a key to a new area in the overworld. Iirc there's optional dungeons too from which you just get some new weapons. I think theres around 10 dungeons total and 8 areas, I'm not too sure though.
The game is easy at some points but usually its just a hard son of a bitch but not in any cheap way so its all cool.