I actually like the Watchtower idea. In fact, I am astounded we can't build any field fortifications, watchtowers, camps or whatnot, and are completely limited to founding a new city, which then takes a whole 15 seasons to build just one hedge wall. How about a much smaller army camp you can surround with stockade within a few months? I want my border forts!
And watchtowers, definitely. It makes no sense having them only buildable from a city... you don't always need to found an entirely new city, because a watchtower isn't a civil structure in any sense of the word, it is strictly military and it's purpose is to watch over an area of land. Historically empires and kingdoms would build entire networks of watchtowers, which acted as checkpoints, waystations and naturally, border watch. Men at a watchtower would watch the land at all times and if they saw an enemy army approaching they would send back runners/riders to warn the military so they could mobilize and meet them with early warning. Watchtowers might also have an increased line of sight if placed on a hill, and have a lot more sight over plains than they would through forest. It just makes so much sense, especially since we don't need to build as many cities as we did before anymore.