Fantastic game, 10/10.
I think you should add a 2x speed button and more towers, like a tesla-tower with a high fire rate, low damage, and low range, but the electricity jumps between enemies. Also, maybe the rocket tower could fire homing missiles at tier 3, giving it anti-air capabilities.
(Hey, I thought it would be cool to add an upgrade option for the missile silo, but instead of upgrading it once, it permanently upgrades all silos: Tier 2 could be extended range and duration, costing you 500, while tier 3 could have higher damage and longer duration, which would cost 1,000.)
And there's a weird bug where it spawns hundreds of those tank guys instead of a few. Really pisses me off because I'm still early-game and this crap just wrecks my run.