Great little tower defence. Nice style, very clean.

I'm terrible at tower defence games so I'm a little lost as to the strategy on the 2nd level, but that gives me something to work towards (I assume it's possible ;^^)

Couple of little things:

• The bullet sounds are a little muffled, I had headphones on and I thought someone was hammering something a few rooms away. I realised what it was when the "hammering" got a bit too vigorous. 
• Placing towers is a little clunky in the beginning. There's only one option so it feels like there's more steps than necessary. Makes sense when there's more options later. Suggestion: make the keyboard shortcuts place turrets without having the menu open.
• Needs feedback for when you try and place a tower with not enough resources. It just disappeared without doing anything and I was a little confused.

Thanks @karidyas. All your suggestions are valid.

• Yes, ammo sounds are muffled. I tried to keep them low, so that the rhythmic beats are more prominent than, random gun fire. I've also considered instead of using standard gun fire impact sound, I could use a more pleasant chime sound. Not sure if that will go well, but might try. What do you think?
• I agree that menu Tower menu options get in the way. I'll consider adding a flag in settings that will let seasoned users turn off the menu display and only use keyboard shortcuts.
• Yes, valid point. There was some visual feedback on tower build with no resources, but I removed it temporarily. I'll add something better soon.

Added 3rd level today, with new enemy and new tower. Give it a shot!