Q: How to gain long lasting Alliances in EB/RTW.
A: The campaign AI was always a diffcult thing in RTW, and the main thing (not the only) that influences it is money.
If you have an alliance with another faction (for example your are playing Karthadast and your Ally is Rome) you have always to fear that they attack you out of no reason (as it often happens). To make the Alliance lasting you have to make gifts to them. The best way to do this are tributes. Giving them small amounts (100 - 500 or whatever you can afford ) of money every turn make them stay friendly is working well. And after some time your Ally will be offering Gifts (money) to you too.
will it also work if you have a direct boarder with a faction you're paying tributes to?
thanks