ShiroPro:RE Collaboration Koga Castle Goshuin (with 110-yen stamp)

This is a castle stamp made in collaboration with Furuoka Castle, which appears in "Oshiro Project: RE ~CASTLE DEFENSE~", a popular tower defense game with over 4 million registered users distributed by DMM GAMES. It comes with one 110 yen castle stamp-shaped postage stamp that can be used for standard mail.
It is postcard-sized.