+100% settlement defense.
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A Fort also reduces the chance of a settlement being raided by natives.
While the European cities of South and Central America were in large part built upon the ruins of native settlements, many cities of the North American frontier began their days as forts. More than simple stockades, forts were large enough to house the workings of entire cities within their protective borders. Often planted as outposts in frontier territories, many forts became centers of economy for the North American interior. Such was the case for Fort Pitt and Fort Quebec, both of which blossomed into major metropolises. Today Pittsburgh and Quebec City remain among the most influential metropolises in their respective regions.