MY last idea anyway. Anyways it's something that's been done before in a previous online game from SE. Ever heard of Fantasy Earth Zero? Long story short, Fort Defense/Siege. Its not a difficult concept to perceive. Look it up, you have your resources in front of you. Oh right, here's a link to the game modes: http://fanearthzero.wikia.com/wiki/Chronicles

They're even split up into phases. Why not use this??