8-man is a full party, 24-man is a raid. It's not the same thing and not the same content.
The Crystal Tower is an alliance raid. It’s content that is taken on by an alliance of three parties, for a total of 24 players.
The Great Labyrinth of Bahamut and the Crystal Tower are similar in that neither of these raids will be completed in one visit. We will continue to add floors over the span of about a year and a half.