The rationale behind the use of forts in Britannia and Britannia Second Edition
I am going to try to explain the rationale behind the fort system used in Britannia, but please understand that nearly 25 years after I designed it, I have absolutely no recollection of why I did what I did; I am giving my best guess about why I did it!
I wanted to somehow indicate how well the Roman was doing at defending "civilization" from the predations of barbarians. Something had to represent success or failure in each area. I used the forts to represent all this, as well as to represent the general defense capability of the Romans. While the Romans could not be everywhere, they could move quickly from place to place, so the fort represents their ability to bring defenders to a particular location. The Romans did not actually build forts all over Britain, nor did they avoid building forts where Britons submitted to them. The fort is a simplification.
Hence, forts cannot be rebuilt, because destruction of a fort represents a general failure of the Romans to defend an area. Yes, in the real world, they could and perhaps would rebuild forts after retaking an area. But then we'd have no record that the area was ever ravaged by the barbarians. So I chose the simple method of leaving a destroyed fort in an area to mark the failure.
I cannot say why I did not choose to leave a destroyed fort marker there, yet let the Romans build another fort. One can guess that I was trying to keep the piece count down. Or maybe, because I was obviously dealing with an abstraction, I wasn't bothered that the fort could not be rebuilt.
Today, I use other methods. In a game that is "Britannia as it might be if designed in 2005", I have forts in every Roman area including the submitted areas. Still, in this smaller,simpler version, when the fort is destroyed, it represents the failure of the Romans to defend the area, they cannot rebuild it, and they get no Limes points for it.
In "MegaBrit", intended to be much bigger and longer than Brit,I have separate "Control Markers" to track the success or failure against the barbarians. Forts are built where the Roman chooses, with a limit (depending on the number of areas he holds) to how many can be built each turn (about 4 per turn on a board of nearly 60 areas including Ireland). Destroyed forts can be rebuilt. This is more complex, but more "realistic".
Perhaps in a future version of Brit I'll consider using this MegaBrit method; but I don't want to make Brit much more complex than it is now.