SAO: Aincrad

A huge castle made of stone and steel floating in an endless sky.
That was all this world was.
It took a vagarious group of craftsmen one month to survey the
place; the diameter of the base floor was about 10 kilometers - large
enough to fit the entirety of Setagaya-ku within. Above, there were 100
floors stacking straight upwards; its sheer size was unbelievable. It
was impossible to even guess how much data it consisted of.
Inside, there were a couple of large cities along with countless
small scale towns and villages, forests and plains, and even lakes. Only
one stairway linked each floor to another, and the stairways existed in
dungeons where large numbers of monsters roamed; so discovering
and getting through was no easy matter. However, once someone made
a breakthrough and arrived at a city of the upper floor, the «Teleport
Gates» there and of every cities in the lower floors would be connected
making it possible for anyone to move freely through these levels.
With these conditions, the huge castle had been steadily
conquered for two years. The current front line is the 74th floor.
The name of the castle was «Aincrad», a world of battles with
swords that continued floating and had engulfed approximately six
thousand people. Otherwise known as...
«Sword Art Online (SAO)»