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.
Towers actually have a primal form of sentience. They enjoy being owned and fought over. This intelligence can be awakened into full fledged sapience with an appropriate caster link (a Thinkamancer, a Dollamancer and a Dirtamancer).
Awakened towers do not count as units. They qualify as sentinels and are capable of speech and moving their insides. They have an as yet unspecified set of elemental powers they can use to protect their city. They can manage their own upgrades when authorized to spend the treasury. Shirley has sufficient skill in Dirtamancy to even repair the city of Charlescomm's electronics.Hvs.tCF 281
Real World References
Mage towers are a common trope in fantasy.
The fact that towers are intelligent might be a reference to tutelary deities.