The Fortress is a building which can be built in all epochs since the Copper Age. It has 2000 hp and can garrison up to 40 pop. When a unit is garrisoned inside a fortress, it's pop cost doesn't count until the unit is ungarrisoned from the fortress. In addition, fortresses are the only buildings along with settlements, town centers, capitols and first-time military buildings that you need 2 of them in order to advance to the next epoch.
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