High-rise Invasion

Tenkuu Shinpan (天空侵犯, Tenkuu Shinpan, lit. High-rise Invasion) is a manga series written by Miura Tsuina and illustrated by Oba Takahiro.

Plot
High-school girl Yuri suddenly finds herself on the rooftop of a high-rise building. She's trapped in a bizarre world surrounded by skyscrapers, where a masked man cracked open a man's head with an axe before her eyes. She makes fast friends with another girl, Nise-chan, and they traverse the world together, seeking a way to end the mysterious realm.

Official

 * Mangabox