Built by Settlers only. Press the Fortress key (F key) or choose the Fortress option from the Orders menu. Doubles the strength of defenders. Units defend one at a time. May not be built or exist in a city square.
Civilopedia entry[]
To guard important positions such as mountain passes, fords, or invasion beaches, you may build a Fortress. When properly defended, a Fortress may block the path of approaching enemies. Fortresses may be built by Settlers once you have achieved the advance of Construction. They may be built on any land square except one containing a city. A city founded in a Fortress square destroys the Fortress. A Fortress doubles the strength of all defending units after considering terrain and veteran status. Units defend a Fortress one at a time. Unlike combat in normal terrain, all other defenders in a Fortress are not destroyed when one unit in the square is destroyed.