Red Alert 2 style defensive towers. Prism tower: charges and fires a beam of white light; nearby prism towers link up in a chain and multiply the damage.
just some ideas for earlier turrets for before you go into space, the allied pillbox, soviet sentry gun and maybe yuri's gatling turret also if you ever do tanks could do the tank bunker yuri gets in yuri's revenge as like a customisable turret, being able to throw different vehicles into it.