Like all RTS games, buildings form the very heart of your realm, providing valuable infrastructure to expand and maintain your empire.
Recruitment structures are primarily involved with the task of training units. Although a few of them can function like enhancements and some also host valuable upgrades and abilities, their most valuable function is still the conversion of resources gathered from economic structures into peasants, warriors, and other assistants.
Although their primary task isn't to create more units, some Enhancements, Defensive structures and Economic structures can create units, but usually these units are meant to be used primarily as support units, not combat.
Economic structures are just that — their primary task is to drag resources you can use to either create more buildings, or create minions from them to impose your will and ultimately win the game.
What more can we say?
The main task of Fortifications is to function as point defences to help defend your settlements from attack and kill intruders.
While not directly involved in defensive or production duty, enhancements are vital infrastructure which allow access to upgrades which improve your faction's power — be it economic, military, or otherwise.