I feel like the issue behind seal Bahamut is not due to power, but timing. As has been suggested, Dalamud is a container for Bahamut. In the end, the 12 were attempting to seal him back in a newly formed container. But the one thing that has been failed to be mentioned is THE KEY.

When Dalamud first explodes (clearly seen at 2:49 in the video) the "key" or tower structure is released.

At 4:58 in the video, the 12 runes/glyphs are powered, and they create a similar structure to contain Bahamut. As they are finishing this process, Bahamut bursts from the container, BEFORE THE KEY LOCKS IT.

As stated by Yoshi-P in the Live Letter:
"Q20: Please tell us more about the Crystal Tower and the Great Labyrinth of Bahamut!
A20: The Crystal Tower will be an alliance-specific raid and an alliance party consists of three parties. Both the Crystal Tower and the Great Labyrinth of Bahamut will be expanded in multiple stages.
Also, as far as the Great Labyrinth of Bahamut goes, you probably figured out the Dalamud was used to imprison Bahamut. Players will find out why Bahamut was trapped, why the key-like object fell to the ground, what role the key plays in the dungeon, etc. Sorry, this is much as I can say for now!"

So, my guess is that we will somehow use the key to reseal Bahamut in the structure provided by the 12. I am interested in how the Great Labyrinth of Bahamut will factor in to the story.

Thoughts?