The next Economic Policy to enhance Adjacency bonuses for a District, this time the Commercial Hub. Like the Harbor, its Adjacency bonuses yield Gold, but it is comparatively easier to get good bonuses for the Hub than for the Harbor - it is rare to have more than 2 Sea resources in suitable location, so that you could get a bonus of 2+ for the Harbor; at the same time every tile next to a River will yield a +2 bonus for the Hub, and with little effort you could get 3 and more bonus. That means that, most of the time, it is more profitable to use Town Charters than Naval Infrastructure. The latter one, however, is available earlier; and anyway there are yet other possibilities to enhance your Gold input - you should always think of which one exactly will yield best results for your particular empire.
Civilopedia Entry Edit
Traditionally, the granting of a town charter gave it and its inhabitants the right to certain privileges in the feudal system. Townspeople were burghers, as opposed to serfs, and “free” in the sense that they were protected by (and owed allegiance only to) the king rather than the local manor lord. Charters also gave the burghers some semblance of self-government, in that the rich merchants would be allowed to choose a mayor and hold a market or faire under their own auspices … so long as acceptable to the king, of course. As early as the 10th Century AD, German towns were being granted town charters, with their own militia, courts, and taxes … the model for all those after.