http://gate-thus-the-jsdf-fought-there. ... here!_Wiki
Gate is a Light Novel, Manga, Anime series where a Gate opens in Japan and a Medieval/Romanesque army comes through and attacks, the JSDF immediately counter attacks, etc.
The series is heavily debated on whether it is any good, or if the author wanks the JSDF, etc.
But everyone usually agrees the premise of the modern world interacting with a medieval fantasy one is an interesting idea.
And I like that as well, and I would love to RP that.