The Empire Strikes Back was released by Parker Brothers for the Atari 2600 in 1982. It shouldn't be confused with the vector based arcade game that was released in 1985. While the Atari 2600 version is somewhat derivative of Defender, it is a decent game and one of the better Star Wars games on the 2600.
The basic game play is much like Defender in that you can fly horizontally in either direction and shoot things. The difference is that instead of shooting down flying saucers and rescuing humans, you are trying to destroy Imperial Walkers. As the game progresses, they move faster as they try to destroy your shield generator. It is a little more simplistic than Defender and as such doesn't have quite the replay value.
Empire Strikes back is a fun enough diversion in small doses but it can get repetitive. Fortunately, it is easily emulated and original cartridges can be found reasonably priced, particularly if you aren't concerned about a complete in box copy. I don't believe this one has been re-released and as a Star Wars movie license probably won't be. However, there is also an Intellivision version of this game if you prefer that system.
Screen shot above is from the Atari 2600 version of the game.