Haven – "Everything is mixed up, truth and falsehood" [Campaign 3 – Mission 2]

I need help with the walkthrough for mission 2. I've been trying for days and can't seem to manage it. I completed the Necropolis campaign without any problems, but here, on the second mission, I'm stuck. The problem is this: by the end of the first week, I capture the Necromancer's castle, and by the middle of the second week, I capture the first castle of the Haven faction, right across the river. And that's where the first problems start: I can't get past two bridges – either enemy heroes attack (and the first one attacks just two days after I capture their castle), so I'm constantly defending, or the main objectives always have a "deadly" status (their troops grow rapidly). I'm playing on the normal difficulty level. Thank you in advance.