"Fortress" is the generic name given to massive fortifications which can garrison troops as well as increase your borders. Aside from this, they are also the pickup point for key units, most notably Generals and select elite units. The most notable military unit to be trained here are the War Elephant lines, trained by the South Asian factions — India, Vietnam, Siam, Burma and the Malays.
Unlike other mods and Rise of Nations itself, the many defensive structures that make up the Fortress line do not upgrade to each other. Instead, they are one and separate, and in order to reach the next one you must research the maximum level of Fortification available to obtain the next Fortress-type building.
- CASTLE: The primary fortress-type structure, it is however very weak, and can only be built near population centres, making it very limited in its use.
- FORTRESS: This massive building is clearly a level higher than the Castle, and can be built anywhere.
- CITADEL: The most powerful of the three, it is also the largest and the most expensive, but is capable of untold devastation and is also armed with armour-piercing guns.