Fortress (Zoroaster)

A fortress is an isolated defense installation. It may be located in a city, but it is always self-contained. It provides the most secure location to lodge troops, store grain and valuables, and to house nobles. It does not really help defend the province it is located in, except in that troops can sally forth from the fortress, forcing invaders to detach besiegers.

A fortress is almost impenetrable, and can take years to reduce in a siege.