Forts are indispensible. Try playing RTR as Gaul when you are attacked from all sides (Britons/Germans/Dacia/Romans/Iberia/Carthage), being able to block or slow down attacks is critical to the defense of Gaul.

Even in RTW 1.1 the ai seiged my forts and sometimes took them
though I then usually seiged them back when my armies had time to arrive. But the critical thing was I never lost a city. Looking forward to see how much ai has changed in RTW 1.2( RTR 5.0 works with RTW 1.2 though I've yet to push it far)