Update: Star Guard is a 2010 IGF finalist in the "excellence in design" category.
"Guide the spaceman through the castle and defeat the wizard" -- can't get any simpler than that. Do you need any more reason to blast your way through nine levels of old-school goodness? Didn't think so. Star Guard is a refined, minimalistic take on those 80s run-and-gun classics; like its predecessors, it focuses solely on fun and hits its mark (in the face with a laser beam, I might add.)
Developer Vacuum Flowers surgically applied Occam's razor to every aspect of design, and the high level of polish pays off. The pixel art and sound effects would feel at home on an Atari 2600, and would have been all the rage ten years before I was born. The game gives you the same verbs the first two NES Megaman titles had, and works quite well under these constraints. The enemy and level designs play off each other nicely, and blasting away baddies is...well, a blast. The only deviation from the norm is its difficulty. Not only do you get infinite lives, but when you respawn you turn invulnerable for a short period of time and can plow through enemies and traps. If you yearn for a challenge, though, just give Hard Mode a shot.
Nothing more needs to be said really. Have fun.