In principle it should be possible, but aside from being extremely expensive, it would be technologically challenging. A massive robot as tall as a skyscraper would have to be structurally stable and strong. That means it would require some kind of endo- or exoskeleton that holds it up. Together with the robot’s working parts, this would need to be made of material that is robust enough that it wouldn’t buckle under its own weight. Yet, it would also have to be light so that the robot can counter the forces of gravity and move around easily – especially if it has to defend the planet against alien monsters! There is also the issue of its power requirements. Plugging into the mains is obviously not an option so the robot would have to contain some kind of huge battery or fuel supply. That would further add to the weight it has to carry around.
Answered by Michael Simpson