The Rights of Man footnote 29
It is difficult to account for the origin of charter and corporation towns, unless we suppose them to have arisen out of, or been connected with, some species of garrison service. The times in which they began justify this idea. The generality of those towns have been garrisons, and the corporations were charged with the care of the gates of the towns, when no military garrison was present. Their refusing or granting admission to strangers, which has produced the custom of giving, selling, and buying freedom, has more of the nature of garrison authority than civil government. Soldiers are free of all corporations throughout the nation, by the same propriety that every soldier is free of every garrison, and no other persons are. He can follow any employment, with the permission of his officers, in any corporation towns throughout the nation.
Thomas Paine homepage