The MMO-Champion section on Flame Leviathan lists that hard mode is engaged when towers are destroyed - the following text: "The Hard Mode of this encounter is when you destroy the towers in the Area prior to the fight. Each tower destroyed will give the Flame Leviathan an unique ability based on which tower you destroyed."
In Wowwiki's Flame Leviathan article, the exact OPPOSITE is indicated - the following text: "Players who choose not to destroy towers up to Flame Leviathan will have to deal with their unique abilities named after the four Watchers in Ulduar. These towers will target the ground and place pillars of light colored correspondingly to their spell school. Direct contact with these beams can and should be avoided at all times."
It makes more sense (at least to me) that destroying the towers would be the regular mode, leaving them up would be the hard mode. In any event it looks like some facts need to be reconciled between these two sources.
Regards,
-Thedrun