It's actually quite well-made as a game with adorable assets and graphics as well as fantastic sound design (mortar is a bit annoying after a while though) that makes this game to be pretty satisfying. However, it's pretty unbalanced in the player's favor.

Getting miners early on can carry you really hard and even later, they can slow the gigantic enemy hordes. The turrets can eventually get to be pretty good but they won't actually carry the entire waves. However, the strongest asset you have is your minigun if you focus on the critical damage. It can just insta-wipe even late game (wave 50 for me) waves within 10 seconds of waving it around the screen. Throughout the game, no enemies touched my wall aside from the flying yellow baby heads that I couldn't hit with my un-upgraded minigun. The turrets have aimcheck though so that fixed that.

I found the hydroponics to be pretty unneeded due to gold generation just being easier to get from just actually killing the enemies (and miners help out a lot). Gold also becomes useless around wave 20 or 25 because you got all of your upgrades. Additionally, the riflemen were too expensive to get early game but had no range to do anything except provide lag in the late game. Pretty cool having a standing army on your wall though.

Some upgrades are useless such as turret turning rate, miner speed, auto-minigun shoot (kinda, it is useful but late game you should just swipe the screen occasionally to remove lag and continue to next wave faster), the orbital strike upgrades, and the radar which didn't seem to do anything.

However, I still did enjoy this game. It's the actually cool wave defense game where the enemy ARE waves and they not only get stronger but more numerous. This gives lag problems (which slowed my input speed for upgrading turrets very early on) but it's a worthy sacrifice for a screen full of zeds. I think if you as a dev really want to improve the quality of the game, make the other buildings and some upgrades/techs cost gold in upkeep which the hydroponics can somewhat alleviate. However, the player should not be able to be self-sufficient with hydroponics at all and thus force them to increase the wave for more enemies thus more gold which means they need more tech/upgrades which raises gold costs, etc etc. Essentially, the hydroponics become upkeep reduction (which, imo at most, should cap at -50%)

Critical chance should also be severely nerfed because even 10% is really high as you can shoot over a thousand bullets a second. I don't know how much extra damage critical is but it's a lot. The damage should also get nerfed pretty severely imo. 

These two changes probably won't fix the fact the player is never in danger BUTTT it'll help out. There's more work to be done there in the fine details of grinding out tweaks and changes.