 If you liked
Isekai no Seikishi Monogatari
|
 ...then you might like
Break Blade Movie 1: Kakusei no Toki
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both have guys with incredible talent using some sort of machine that runs on power provided by the user to help save their country from being taken over.
> Mecha battles in fantasy world.
> Protagonist is a mecha pilot with great capabilities.
> War between monarchical countries.
> Technologies of Ancients. There was a civilization that was very advanced many centuries ago.
> Golden-haired princess heroine.
Break Blade series are movies and Isekai is an OVA ( 13 Eps and 40 min Duration)
Both are about Robots fighting and saving or fighting a force