When I was 15 years old and spent 3 weeks in Lancashire, there was an arcade game that I liked at the local pub. (That pub didn’t care that I wasn’t 16.)
The game was a lot like Galaxian but had another twist. I the later stages the attackers could drop “spies” that could then attack you from behind. I thought the game was called Alien Spy. Maybe the words “Alien Spy” was printed on the arcade cabinet.
I only found this game one other time. At Tumba-tältet (that was an indoor golf driving range) in Stockholm. Since then I have not seen it.[clear]
Using my Google-fu powers with only that inaccurate name of the game as my only clue I finally found it. It is called Uniwar S. Using an emulator, I could also play it.
The first level is a lot like Galaxian except that all attackers look the same. If you know how to play Galaxian, the first level should be quite easy.