Oh...
Then try creating a unit that has the model of a tower. Using normal units as towers does also prevent possible server splits.
And in order to let them appear even in the fog,
turn on "Use Extended Line of Sight".
As for the minimap, if you want buildings to be displayed that much:
Try creating a dummy structure with a collision size of 0 that like, does nothing, at the spots where you have the "towers".