Aincrad
Aincrad is the flying castle at the centre of the Sword Art Online universe, a hundred-floor structure that the original players of the MMO were trapped inside when the death game began. Echoes of Aincrad depicts only the first two floors.
Scope in Echoes of Aincrad
Series producer Yosuke Futami has said that fully rebuilding all hundred floors would take roughly ten years of development, which is why this entry focuses on the opening two floors at high fidelity instead of a shallower pass at the full castle.
Floors in the Game
Floor 1 - Town of Beginnings. The starting floor, with the Town of Beginnings as its hub.
Floor 2 - Urbus. The second floor, with Urbus as its hub.
Clearing a Floor
In the Sword Art Online fiction, progressing up Aincrad requires defeating the floor's boss. Echoes of Aincrad preserves that structure, including a confirmed fight against Illfang the Kobold Lord on Floor 1.