Babel Tower is one of a series of commercial complexes on the outskirts of the Tokyo Settlement, administrated by organized criminal elements as a casino wherein human trade and other illegal practices are rampant.
Background
Lelouch first appears in this building to gamble against the Black King who he defeats in Chess. After the Black Knights appear to retrieve Zero, he recovers his memories and then, directs the Black Knights to destroy it, simultaneously killing the current Viceroy Calares and making a direct path to the Chinese Consulate.