Back during the PS1 days I had Defender on some kind of retro arcade collection disc. While most of the games on it were rather lame, I did like playing Defender.
Since it came out 8 years before I was born in 1980, it does look rather basic. The main point is to fly around in your space-ship shooting aliens and rescuing astronauts about to be abducted by UFOs.
It's not the greatest game to be sure, and it's way too difficult but I like it.