I can purchase 2 versions of the game: The Tower of Eternity and Assault on Balaurea.
One site lists Assault on Balaurea as an expansion but the NC Soft page only sells Assault on Balaurea. Is The Tower of Eternity required?
By purchasing Assault on Balaurea, you'll get an account for Tower of Eternity. It is an expansion, but just purchasing the expansion gives you all the content in the original game - you are not required to already have the original game.