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 civilians, warriors, and other assistants.
Although creating units is not their primary task, some Enhancements, Defensive structures and Economic structures do create essential support units. Markets can produce Caravans and Merchants, which are typically unarmed with the exception of the Dutch Unique Market units. Spies and Generals, which serve espionage and military support roles, respectively, are produced at Forts. Universities produce Scholars, but under normal circumstances, these units should remain garrisoned until their building is destroyed. Additionally, nations that have researched at least one Government will have their Government's Patriot respawn at their Senate.
Economic structures form the bulk of your city's economic output, notwithstanding nation powers that provide additional resource income. These buildings constitute the primary source of Food, Timber, Metal, Wealth, Oil, and Knowledge as soon as they become available, although a significant Wealth income is also provided by Caravans and from Temple upgrades (proportional to your controlled territory).
Some economic structures are terrain-limited, and can only be built on map-generated structures. Mines and Woodcutter's Camps are placed on Mountains and Trees, respectively, and their maximum capacity is proportional to their size. Because these buildings can be quite expensive in the early game, it is most efficient to build them on large structures if the map permits this. Oil is terrain-limited, but both land and sea spots always supply a single structure with no exceptions.
Other economic structures are city-limited, and only a finite number can be built anywhere in each city. While the University and Market are restricted to one per city, civilizations are typically able to build five Farms per city, while Egyptian players are able to build seven.
Rare resources also constitute economic structures, and provide up to two resource incomes that increase with [tech name] upgrades. They also provide a unique effect that can indirectly benefit a nation's economy, such as Salt's Barracks troops cost reduction.
Fortifications serve as point defenses to help defend your settlements by killing or otherwise deterring intruders. Initially effective against infantry units, Fortifications take on a less involved role in later eras by providing a means to capture territory, and by the Industrial Age their primary focus becomes defense against highly destructive Airbase units.
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.