Walkthrough tips, Discussion of Campaigns [Heroes 5]

I think I read it carefully, but I don't remember seeing that kind of bug described. There's a level where you need to hold a castle on an island, and demons constantly attack from a portal. The hero's task is to swim across, free two imprisoned heroes, and capture the Necropolis castles... I've already killed everyone I could there, but when I approach the prison with the weak heroes, 9999 dragons immediately appear as guards :(((
I have no idea what to do with this, to be honest, and I've even stopped following the walkthrough... but I still want to finish it! :confused: