Tank Knights Portriss

In a raving world, legendary knights stood up. Will they become gods of war or world saviors? Three Portriss Knights and sole human boy are now going to change the World! Portriss Planet, home of where robots live in peace. However, this peacefulness was ruined by the sudden appearance of a Dictator "Dark Portriss". Dark Portriss plans to attain infinite powers through the use of the "Space-time destroying Cannon". The only ones able to stop this are the Portriss Knights! A boy, Yuuma, accidentally got into the Portriss World, and the Portrisses` adventure begins! (Source: AniDB)