How will it effect non-computer players?
I suggest the spell "illusion" to create a fake non-moving copy of an existing unit. As a result, it will also increase the combat rating of the town.
Also, "invisibility" will do the opposite and hide a unit, reducing the combat rating of the army. As an option, movement of the unit might dispel the enchantment.
A third spell can be "true sight", that will make the enchanted unit see both the illusions and the invisible units.