Sword Art Online Aincrad - VOL 2

An impossibly huge castle of rock and iron, floating in an endless expanse of sky.
That is the entirety of this world.
A tireless, month-long survey by a team of fanatical experts found that the base floor of the fortress was more than six miles in diameter, just large enough to fit the entirety of Tokyo's Setagaya ward inside. And considering the one hundred floors stacked one atop the other, the sheer vastness of the structure beggared the imagination. It was impossible to estimate the total amount of data it all represented.
Inside the castle were several bustling cities, countless smaller towns and villages, forests, plains, and lakes. Only one staircase connected each floor to those adjacent, and these staircases were located within dangerous labyrinths filled with monsters. It was difficult just to find them, much less reach them, but once someone had cleared the stairs and arrived at a major city on the next floor up, a teleport gate linking to the new floor would open in every city below, allowing all players instantaneous travel among the various tiers of the castle.
It was thus that over two long years, its inhabitants slowly but steadily conquered this giant fortress.
The castle's name is Aincrad, a floating world of blade and battle with about six thousand human beings trapped within. Otherwise known as-
Sword Art Online.