Sounds like a good idea to make the siege part a bit more interesting, but if we are to strive for historical correctness, it's really not that appliccable. At some points of their frontier, the Romans did have an organized defence, as described by Edward Luttwak in his "the Grand Strategy of the Roman Empire," where they organized a "defence in depth", where they had fortified grain supplies, strategicly placed observations etc. However, modern scholars believe that this did not represent the overall situation. Territory was not concieved in the same way as we do today! It was more "spheres of influence", and it was often hard to decide actual borders. The Romans used the rivers of Germania and Dacia as borders, mostly because they were more easily defendable (Actually, they called them "frontiers" not borders) . But there was no way they could control the entire frontier. Ammelianus Marcellianus tells us about an event where the emperor, moving down from campaign in Germania, came across a city, that to their surprise had been occupied by barbarians! "Border incursions" were frequent, sometimes they were even recognized by the emperor, but often, they never learned about them before rumours started spreading of smoking villages in certain regions.

When barbarians repeatedly sacked Rome in the late empire, they often came upon the city with surprise, moving down all of Italy, almost uninterrupted.

I believe that the current system of watchtowers already gives us a more than adequate "early warning system", which is a lot more than the Romans ever had. It also appears to me that the time for preparation etc. already is allowed for in the number of turns a city can survive. Besides, the siege mode in this game is so full of bugs already... do we really want to add more to that?