I've been playing with Rome and I have several cities connected by roads and harbors. For some reason, after so much time, I get a message that the "trade route between XXX city and your capital has been broken". There is no indication as to why (all of the roads still exist, as well as the harbors... and no enemies are blocking the routes). Has anybody else experienced this? Does anyone know what is causing it and how do you fix it? Is it a bug or gameplay glitch or part of the mechanics?