Necropolis – “Spider’s Trick” [Campaign 2 – Mission 4]

Hello! I can't win; I'm capturing my second castle on the surface, and then 5 heroes attack in Schism. After that, dozens of Inferno heroes attack my castles, trapping them in one castle and constantly besieging them. Can you advise me on what to do?
And if I destroy a castle in Schism, will the growth rate of Inferno's soldiers decrease?
Please help, answer me.