Fortresses are tile improvements in Civilization II. Once your civilization has discovered Construction, Settlers and Engineers have the ability to construct fortresses. Fortresses can be utilized to defend city perimeters and to block key points of access from enemy armies. A properly manned fortress can provide an effective defense by doubling the defensive strength of all units stationed within. Unlike normal combat, units stacked within a fortress defend and are destroyed one at a time in battle, rather than being destroyed simultaneously. Fortresses themselves do not suffer damage in the attack.