I really really like this, and had a great time playing. I love the idea - it hits that feeling an idle game (cookie clicker, etc.) gives you while also adding in more interactive, "game-y" elements. There's so much potential to build this out some more...I think you have a successful idea here.
My two pieces of constructive feedback from my playtime would be:
- I feel like the player character blends into the buildings created, making it hard to keep track of where you are. Making the buildings or the character a more contrasting colour would solve this.
- Once I upgraded my Defender once, the rest of my time playing felt quite easy (I didn't feel the need to purchase any more upgrades). A revisit to the dificulty curve for future iterations may be required.
Really good job though, and well done! :D