How do I capture Bari for the second time?

In the Bari campaign, which is part of The Forgotten Campaigns, I am currently stuck on the second mission.

It all goes well until I take control of the camp in the south-west of the map. From this point on I am frequently attacked by rebels and despite my best efforts my forces are usually hardly strong enough to hold off the attacks.

This goes on for a while until I am all out of gold (even the mine to the very south of the map) and basically have to give up because I can no longer build units.

So what is the trick to getting rid of those annoying rebels? I usually manage to destroy their camps after some time but their army never seems to take a significant hit; they always keep coming at me and wearing down my raiding parties on their way to the next camp or to Bari itself.