Xanatos 
I just added the most ugly build menu in the history of gaming, but it does work so it's fine for a prototype I think.
I just finished the state transition between the build and wave phase.
I also added a early piece of code to place buildings on the grid using the scrap resource.
I also did add a shader to show if you can build there or not.
Really really tired now.
I started to implement some turrets for my current game idea yesterday. I did complete my experimental phase to check out the navigation mesh and tile map.
Now I start to get one part of the game loop into a working state. The turret does attack the Godot icon based on a threat algorithm (lowest hp, closest, ...) and deals damage to it based on some entity stats and stat modifier.