Walkthrough tips, Campaign discussion [Heroes 5: Hammers of Fate].

I'm talking about the moment when Godric's offspring is left alone.
I don't understand what's going on or how...
You can't bring troops there – you have to leave only 1 unit and then go to the city. But then you have to fight, and there are too few troops on the available area (between the gates and the castle) to defeat the red creatures and go into the dungeon. They actually do quite well there – around fifty of them – 30-35 can still be killed, but the rest are a real pain.
I was hoping for the Dungeon (thinking there would be a friendly warrior with the right number of units) – but no, there's nothing there except for another quest (collect a hat and kneepads).
Guys... please help me out :)