I'm looking for an old anime which I watched when I was young, during the 90s. As far as I can remember:
- The earth is flat in the story
- The storyline is about this protagonist trying to save the world by killing monsters by transforming into different mecha?
- The protagonist transform by inserting a disk (?) into a device on his hand
- At the end of the first arc, the world isn't restored, and the protagonist had to return all the disks he collected to their original place
- Each time a disk is returned, one of the bad dudes will turn back to normal
- To make things clearer, it is the protagonist himself transforming