I tried something similar with Pontus, I expanded very little and played defensive for decades untill finally the Seleucids brought down their hammer of doom on me and stack after stack after stack fought me to the last soldier.

I guess in BI this is what's probably going to happen to the western Romans, if the AI has any competence and you don't make any smart moves it should be an "Alamo" campaign where you have to shuffle armies around the world to try to keep your land and eventually retreat to Italy to avoid a sack of Rome. I hope this isn't only wishfull thinking.

RTW can be fun both in victory and in defeat, I sure had plenty of fun last time I lost as Germania in RTR6.0, I did put up a good fight, I over-extended myself and I suffered from my own mistakes, but I went down fighting, if only Germans could build stone walls...