Relive the classic, kind of, and in 3D. Defender has all the element of home (home being the original 80's) title with quite a bit more depth. Speaking as one who worked on this title I can give some fair insight on this title. Visually the movies, crafts, and the enemies are a treat, but the landscapes and lack of variety in structures can get tiresome. Gameplay though on this one is pretty tight, and coupled with a great control scheme (using the alternate setup) it can get someone down to play comfortably for a few hours.
Many levels are thrown into this game, many can be optional, but that is up to the player. As you progress you'll find that you could squeeze by doing a mission per planet and win, or try doing up to 4 on each which adds a lot more story and more perks. Perks come from the credits acquired by the points earned per mission. These points can be used to buy an extra life, new guns, and new bombs for a particular craft, of which up to 6 you can eventually earn. (Tip: I suggest holding your cash and dropping it all on the Defiance when it's opened up as it will make for the best craft in the game.)
Level designs are a bit generic as you'll have to do escorts, defense, offense, rescue, and a bit of research too. Overall it's not as bad as that could sound as objectives can and will just pop up at times when you're playing which can throw your plans for an interesting loop. I suggest with the price the game currently runs for to check it out. The story is fun, the ending is a shocker and could be depressing (not in a 'this game sucks' way) for some. I suggest giving it a try, but be aware, stay away from the multiplayer...whatever the version of the game you play it stinks and can be considered nearly unplayable and definitely not fun.