Andrey Rudenko, a contractor in Minnesota, has completed his 12-foot-tall concrete castle made with a 3D printer that he built. Rudenko developed a concrete-spewing extruder that doles out layer after layer following a programmed pattern. To get the right mix, he used cement, play sand and additives. He now plans to build a home with his invention, saying, "not just someday, [but] I can print a house in like a couple months."