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The following buildings can be constructed in a Commercial Hub:
This district is your main economic booster. Historically, water trade has always been more effective than land trade, and this district's adjacency bonuses show this marvelously! A Commercial Hub on a River or right next to a Harbor is a must-have combo in any empire.
Note that the description "+2 Gold from each adjacent river" is incorrect - the district gets +2 Gold for being adjacent to at least one river. For instance, if you put it on the upstream tile where two rivers merge, the bonus will still be only +2 Gold.
There isn't much more strategy involved with this district than the place you put it. It will simply help your Gold flow.
The "commercial hub" of a city usually refers to the downtown business district, where capitalism and greed enliven the city's finances. Here are the markets, banks and exchanges that make a few folks rich and fuel city growth. In time, evolving from a simple collection of local financiers and corporate offices, a commercial hub might even grow into a "global financial center." There is even a system for ranking such global centers; until 2013 London held the top spot on the Global Financial Centers Index, when it was supplanted by New York City, both closely followed by Hong Kong, Singapore, and Tokyo. The quest for profit is obviously a universal trait of civilization.