Your new secret weapon: STARGATE, a powerful stellar gateway to the universe. It's the only hope for your beleaguered planet! Once again, you take to the skies in your ship -- now fortified with the mysterious cloaking compound, INVISO. With the help of the STARGATE, maybe -- just maybe -- you can save the last humanoids and wipe out the aliens FOREVER!
Stargate is, in my opinion, Defender "done good." Atari created Stargate as a sequel to Defender. Those who recognize this game as Defender II are correct. Star Wars action figure manufacturer Kenner sued Atari over the name Stargate as they had a product out with that trademark. So Atari renamed the game.
The graphics on this game are near perfect with the Arcade version of the original Defender. From the orange hills, to the better shaped, coloured, animated, and not-so-flickery enemies this game is visually leaps and bounds over the original defender in terms of staying true to the arcade. The game even features a title screen which Atari had been putting in more and more games. The graphics are perhaps the most important aspect of Stargate/Defender II.
Defender is your classic shoot-'em-up "shmup" and as an arcade port, Defender and Stargate are pretty similar. You fly a ship, must protect the people from being abducted by Alien invaders. This is what makes Defender/Stargate unique from other games of this type, as you actually get to play a defensive role in this game defending the unfortunate populace.
The aliens are also interesting, the more that successfully abduct people turn into an improved mode of ship, or mutant. If you manage to destroy the alien abducting a person, your ship has to save them from falling into the ground. Just touching them with your ship will do the job, and then you must land yourself on the ground. Be careful not to shoot them (especially when they are in the process of being abducted!)
Your weapons are the classic laser beam and the of course, the smart bomb. With the bomb, you can clear the viewable screen of enemies. Another of the interesting parts of this game is the radar. Defender is perhaps the first video game with a radar aside of Battlezone (another Atari gem). There is a lot of rich game play in Defender and I haven't even mentioned hyperspace or invisio!
There isn't any music in this game although the sounds are similar to the arcade, which is to say bland bings, pows, and zaps. If you love the classic arcade-y sound, this is a good game for that.
Stargate has some unique features over Defender and I definitely suggest this game over the original Defender. But with a rarity that is a bit over the original (4 vs 1), it may not be so easy to find. Greater variety of enemies, truer to the arcade and just an overall better game!