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A fortress is a region that has been fortified by it's controlling country. For a region to qualify as a fortress, it must have a Q5 Hospital and a Q5 defense system. These regions also need to have a large population, at the very least, 1000 players, to make the defense system worth the expense.
The strongest fortress created so far was the American state of Florida, which, at it's peak during the Great War of 2009, carried 10,000 citizens; the entirety of the United States' population.