Heaven Strada

Aleph grew up listening about rare artifacts and adventures in the ancient ruins. As he becomes adult, he joins his uncle who got position of ruins caretaker.

When he comes to the duel place in the ruins with his childhood friend, Aleph meets a girl in unfamiliar clothes being attacked by inquisition men in black. As enemies are defeated, Aleph takes girl home. She tells that she comes from the world of the blue sky. They decide to guard the ruins together against the men in black and work towards getting to the world of the blue sky.

[From vndbreview]