Defender frightened me, but i kept going back for more, even though it's speed (for it's time), and intensity of attack, especially from mutants, meant it was just chewing up my pocket money - i got a lot further with Scramble (and Moonbuggy !), largely due to it's more sedate pace, and loved the Galaxians derivatives Gorf, and Phoenix.
Wireframes like Lunar Lander and greenscreen Battlezone were available in the cafe at a sports centre we used to visit for school sports, as well as the movie inspired Tron, which i only ever saw, and played, in those 2? school years - great fun, the best times.