Seems odd, because in none of my games have Caesar or Isabella ever went to war with me. In the games where I have seen Caesar, he has been a minor player in world affairs. Isabella always seems to be a major player, but relations with her varied. In one game as the English, she was a real Ice Queen. We never went to war, but she was hanging on to Judaism and I to Christianity. I finally signed an open borders agreement with her and immediately loaded up missionaries on caravels to start converting her cities, only to have her declare Theocracy before my missionaries could get there. Doh.
In another game as the Arabs, I was put on a continent with me in the south, the Aztecs north of me, Isabella N/NE of them, and the Mongols E/SE of Isabella. We were all under Judaism and had good relations, then Montezuma attacked me. I had great relations with Isabella though, in fact, she was the closest thing I had to an ally. So close, that I even declared war on the Mongols just to help her out when Genghis attacked her. After I founded Islam and converted, she still kept good relations with me.
The biggest jerk I've seen so far though is Napoleon. Any game I am in with him, I always end up at war...