Too many words to read =p
Gameplay sounds interesting. Still ain't that clear.
And rewards could be fames gems and shards. Coz these are the most valuable resource in game and being a non frequent game mode the rewards can b pretty high.
And make it once a week atleast.
So basically on my opinion....Its time to make your Heros "Stronger and More Stronger",this "Fortress Feud" event is helpful only when maintain your Heros daily by upgrading
Reason why I told this just bcus, by getting on main point and i.e if your heros in this new event die then your enemy will earn points.