The higher end players have the problem of collecting much more iron than they need. This might change soon after new buildings will be introduced.
I suggest to allow Mystic Tower to be upgraded and each time it would be upgraded, its timer will be shorter and rewards richer.
This could happen once the Guard Tower arrive at the max level. Guard tower requires ham to be upgraded instead of iron. Mystic Tower, could require iron like others building.
The iron would be sane to be used also for upgrading Guard Tower, after all. But this is another story.
Moreover, Mystic Tower work as trading center to exchange food VS iron and viceversa.