Towers have important magical qualities. They provide a bonus to a caster's spells. They also possess a capacity for being charged with spells. Commanders can direct them to target these spells at specific enemy units. If no commander is present, it will target enemies automagically.
A tower is not classified as a type of building. Though impossible to convey exactly why without Dirtamancy sense, it has been stated that they have qualities that make it impossible to imbue them with motion.