The original Defender on the Atari 2600 is rubbish, but Defender II is the shizzle.
Unlike the first game, Defender II has a great feel to the controls, nice movement of the ship, and fast, decent, absorbing gameplay.
You could argue: “why on earth did they ever bother with a Defender sequel on the Atari VCS in the first place?” The easy answer is: because they looked at the first game and thought they could do a LOT better. Which they did.
The result is: arguably one of the best games on the Atari 2600.
Defender II is also known as Stargate in some circles, as per the 1981 Williams Electronics‘ arcade game on which it is based.
More: Stargate on Wikipedia