Really enjoyable
Zombies, defending, upgrades, All the good things we like in games and the design of it all is great. And certainly excellent for your first try. Very promising future.
As a leftie I do have one small request if you make further games where the gamer will have to use both keyboard and mouse together. Either allow the player to customise the keys they want to use or make the arrows usable too along with WASD. Because I don't know about other left handed people on here but I do use a mouse in my left hand which as you can imagine makes using WASD a little awkward with my right. It just don't feel right. And I'm just not as good using my right hand with the mouse.
Not marking you down for awkward controls as I am the one with the awkward hand type :( Just would be nice to have the option in future games.