This is part of the original ideas for the civilization of the British. The original idea was to increase the attack of the towers but I find it much more fun that they are built in neutral territory. This opens up a range of new possibilities.
Originally I was going to put this as a tech, but I think it could be a civ bonus, where it's also a lack of the British, besides, I don't think it's op to need a tech. A turret in neutral territory alone can be easily converted.