Aincrad is a fictional floating castle in the anime and light novel series Sword Art Online. It consists of 100 floors, each featuring unique environments, monsters, and challenges. Players must navigate through these floors to progress and ultimately escape the game. The castle is designed like a massive tower, with each floor representing a different level of difficulty. Players, known as SAO participants, engage in battles, form alliances, and complete quests to advance. The game becomes a life-or-death situation when players discover they cannot log out, making their survival dependent on conquering the castle.