Really Interesting variation on a Tower Defense Game

What is the upgrade path to rocket launcher?

Follow the big gun path (the leftmost option), then pick the rocket launcher instead of the final upgrade.

I prefer Canyon Defense. Tip: don’t neglect to build walls; they’re useful as a means of funneling traffic to high coverage areas, and even more useful as gun platforms right in the path of every vehicle.

And good for making vehicles take a longer route, and even for providing building space for some more of the large improvements. Top tips spoilered: Multiple Recyclers are cumulative in effect - with a lot of them in play you can make an insane amount of money.
Selling and re-buying a Wall Factory makes a section of wall available immediately. You only ever need two WFs at once to qualify for the Laboratory - otherwise you should have one or none.
Doing the same with the Nuclear Silo makes a missile available at once.

My best is somewhere round 280,000 IIRC.