When I was a kid I had these game comp cd's , like galaxy of games and stuff like that. I remember one that had time in the title, I believe, that I keep thinking about and want to find.
It had a top-down view and started with you being in this central room with portals all around. Constantly these enemies that would die with one shot spawned from each portal. You essentially were collecting keys and going from portal to portal, shooting at these monsters that would walk into you and hurt you. I remember the only music was when you first started the game, there was a short light guitar string, like accoustic, not electric. There were sound effects, though.
This has lived in my head forever. I BELIEVE it was in one of the Galaxy of games cd's but I can't find a good list of the games in the older ones.