Hi, it's a nice game and I had fun playing it. That said here are my comments:
Shields are too OP... I got to level 103 and then got bored. From level 7 above I didn't need to shoot anymore, I just spammed shields - since there is large amount of enemies one shield gives you like ten times the shield's cost (so by spending 200 on shield you get at least 2000 just from using it). Altho the giant explosions from shield were awesome at first they got old real quick. Ideal fix would be either not to give player money from enemies killed by shield or make the shield count enemies instead of time (shield would be activated till x amount of enemies were killed by it)
Keep up the good work man