So right now, a solution to protect a planet from invasion is to build lots of avengers. We're talking, like, 300+ of them. This is actually really bad because it lags the game HORRIBLY. SO. 1) We can make avengers... uhhh... lag less somehow. Optimization and all that. Not the best though because avengers are kinda boring. 2) Superavengers: Make avengers 10 times costlier and 10 times more powerful. 30 avengers won't lag as much as 300. 3) Ultimate Anti-orbital unit: Some kind of experimental orbital bot that has a multi-target orbital-only attack. Completely vulnerable to ground-to-orbital attacks and each other, but by having a multi-target attack, the number of avengers you send at it is useless, and it makes it easier to break through avenger shells.
For strictly defensive purposes, Anchors are about 4x (a little more expensive because they shoot ground) and Umbrellas do considerably more damage for less total cost. Both are essentially structures though, so you'll still need enough to cover the planet.
It's for defending a planet that you're the only person on. You land on a moon, capture all the metal zones, build 20 orbital factories, and set avenger production to infinite. I suppose once the Unit Cannon comes up you can send builders to make umbrellas and defensive structures quickly.
