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This is really quite a simple fix/change that I thought of when I was playing the other day.
Promote automatically is casted on the nearest minion, and the minion goes and does its thing.
With Garrison, however, you have to select the tower in order to cast it. Why not change it so its automatically cast on the turret by you? The range isn't great enough to go to another turret, so that isn't an issue.