Finished it! Diplomacy is a powerful tool! I gathered about 1,000 Warmongers, 1,000 Centaurs, 80 Daughters, and 50 Executioners for free, and I defeated the AI before it could get any Free Cyclops or Dark Wyverns. However, one company near the stronghold refused to join me – I had Executioners, but they had leaders, although according to the calculations, they could have joined. I also had a 3rd hero "on duty" near the factory, and he didn't disappear after I captured the 4th castle. Maybe he needed to leave troops behind to capture his own castle. But according to the scenario, the demons will burn it down anyway, right?