I like the pixel graphics, but the lack of contrast made it a little difficult to see. Everything just looked like a brown smudge.
The robots didn't seem to fit in with the rest of the game. I know you were going for a WALL-E type of setting, but the robots didn't match the art style of everything else.
I was pretty confused what the turret was when I started. I thought it was some sort of fire hydrant.
I like the music!
The artifact system is cool and would be great for a larger game to explore the story of the world. In this demo it didn't add to the gameplay.
The game moves a little too fast. I was able to slam down 30+ robots in less than a minute. For an atmospheric apocalypse game with cool artifacts to find, I would prefer a slower pace.
There's a bug where if you are placing a build when you find an artifact your mouse cursor will disappear, making it so you can't play anymore.
Overall: There's a good idea here but the execution needs improvement.