Aincrad

Aincrad is the castle the first arc takes place inside. It floats, it is built of stone and iron, and each of its hundred floors holds towns, fields, dungeons and a labyrinth with a boss at the top of it. Clear the boss and the stairs to the next floor open for everybody. Nobody leaves until floor one hundred falls.

The design is why the arc works. Ten thousand players start in the Town of Beginnings on floor one, and the population sorts itself out from there: front-line raiders, shopkeepers, blacksmiths, farmers, a murder guild, an army that collapses into a bureaucracy. Kawahara spends as much time on the economy and the politics as on the sword fights.

The setting has outlived its own story. Later arcs move to fairy worlds and gun worlds, and the fandom keeps returning to the floating castle, which is the image on most of the covers and the one thing everyone remembers about the series. A rebuilt version turns up again in the games and the spin-offs.