Excellent work
The game was very fun, and I couldn't stop playing until I found the right strategies to beat wave 20, and then there was nothing. I noticed there were achievements up to that point, but no achievement for beating the game? Seems weird. As well, the end is very bland, just a faded "Cngratulations...!". The upgrading system was ok, but many improvements could still be made, such as, previously mentioned, faster tesla regeneration rates, more damage, shields, but also more mini tesla towers, and/or more specific upgrades for the mini towers (range, damage, rate of fire/regeneration, etc).
Anyway, enough of my criticism, it was a great game overall, and I very much hope you make a sequel with more waves, maybe different modes, etc (uhh, here I go again with more criticism even though I said I was done). 4/5, 9/10