I reported the same problem some months ago, and basically, the same responses came in.
The difference was, there was a ceasefire between me as the Julii, and the Brutii, while the Civil war was in progress. Then, Brutii besieged my towns, about 4-6 of them, and I could not sally out against them. The only opportunity I had to do anything at all was when they actually attacked the walls with their rams. Then, I was able to fight back, and I kicked their butts!
One trick I don't think I used at the time was this, and it's a dirty way to finish your campaign, but with just 4 left, and the bug holding you otherwise, I wouldn't blame you if you did.
Train up some diplomats, and bribe besieging armies and they become your own, bribe Brutii settlements, and they become yours. Perhaps bribing even one Brutii army will count enough as an "act of war" to make it possible to attack them. Then your problem is solved, but don't accept another ceasefire with them.
Strength and Honor
Celt Centurion
Bookmarks