Babel Tower

Babel Tower is one of a series of commercial complexes on the outskirts of the Tokyo Settlement, built as part of a redevelopment zone to expand the Tokyo Settlement. It is 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, whom he defeats at chess. After the Black Knights infiltrate it to retrieve Lelouch, he recovers his memories of being Zero and directs the Black Knights to destroy it, simultaneously killing the current Viceroy Calares and making a direct path to the Chinese Consulate.

Trivia

 * Babel Tower is most likely named after the Tower of Babel from the Book of Genesis, which was built a united humanity in an attempt to reach Heaven. God, considering such an act disreepectful, confounded their speech so that they could not understand each other (resulting in th creation of different languages), and scattered them around the world.