It is also one of a few Nintendo Games to feature a female protagonist: in this case, "TGL" features a Guardian, whose mission is to destroy an alien-infested Planet-like space station called Naju before it reaches our Earth. The Guardian must explore the entire station to do so.
What makes this game is so great is the gameplay: it's actually a shoot'em-up videogame with some RPG elements (reminds me of the first "Zelda" in some cases). The player not only must destroy aliens, big bosses etc., but is also able to get new weapons and other power-ups, becoming more powerful as the game progresses.
Another great thing in "TGL" is the Graphics, one of the best at the time. The Sound is also impressive in some cases, but the best part are the BGMs: one of the greatest from NES, and those musics were also very successful among players, and some famous remixes can be found even today (try 'Youtube').
The only bad part of the game is the Password System, too complicated (this was also a common criticism at the time).
"The Guardian Legend" was (and it still is) a nearly perfect NES videogame, a genre bender classic. You can't miss it, and if you like "Zelda" and shooter videogames, you're going to love this one.