Really cool for an "alpha" version! (Already more polished than quite a few games on Steam...)
I'm sure you have plenty of your own ideas on where to take this, but here are some of my ideas:
* UI: Zero and Eight looks very similar in the smaller text, might be more readable if Zero didn't have strikethrough.
* UI: Some minor improvement could be had with upgrades, e.g. displaying a max level (if any), and icons or color variation, e.g. health upgrades green, shield upgrades cyan, etc.
* bug? there is no upgrade between tower level 99 and 100.
* readability? I would perhaps replace "Tower" with "turret" in some cases, e.g. "Turret damage"
* balance: if you're interested in more active play, having different wave types, e.g. more shielded enemies, only kamikaze, all enemies coming from one side but with double HP, etc. Generally the player experience is usually when difficulty goes up and down in small waves, but overall up.
* balance: if you prefer players to have more idle gameplay, I think "random upgrade" option would be neat.
* wishlist: I think one or two more weapons would be nice. Perhaps a laser and a smart bomb.
* wishlist: AI upgrades would be nice, in early levels it would be nice if the turret would consider both enemy distance and turning rate.
* wishlist: more varied upgrades, e.g. drone upgrade, anti-missile drone power, projectile speed, projectile size, spread, shotgun mode, missile turning rate, missile area damage
* wishlist: perhaps limited-use power, e.g. one smart bomb per playthrough, nanobots for healing, etc.
Well, there's more but again I'm sure you already have hundreds of your own ideas. Again, it's already quite fun and pretty polished!