The Mystic Tower is to the lefthand side of the base.  The purpose of the tower is to provide an opportunity to watch the "Mystic Vision", a short advertisement, in return for a small in game reward.  After watching the video there is an 8 hour cooldown before you may watch another.
Mystic Tower

Possible Rewards

Mystic Vision
As a reward you may receive anything from 2 items, up to 5.  A typical reward includes (1) a low level ascension material,  (2) a few gems,  (3) a summon token.

It is possible to receive rare ascension materials, an epic hero token, or epic troop token.  The chances are low, but make viewing the Mystic Vision a worth the time.

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