Transporting an army from one place to another and keeping them well-fed along the way was a difficulty for many civilizations before the domestication of pack animals. I think Ancient Cities should make use of a supply system for individuals away from the town to create the necessity for pack animals for exploration and military expansion. This way there’s no hard cap on a specific date when the player is capable of venturing beyond their own borders. Instead it’s reliant on whether it’s plausible.
In the Americas, the Mayan city-states failed to conquer one another due to the absence of large pack animals. They had to rely on people as beasts of burden. The issue here of course is that people don’t make very good pack animals as they can carry food for themselves during a military excursion, but not anyone else. So each man is essentially carrying his own food.
So food should be a transportable resource in this game and should serve to logistically add depth.