I'm having trouble with the 5th mission in the first campaign. I capture the Dwarven Castle, go through the underground passage, make my way further, defeat the first turquoise enemy, capture the castle, and start developing normally. I defeat the first red enemy, and I'm left without an army. A week later, the second one appears with an incredibly large army and kills me. I've replayed it three times, but it doesn't help. I can't break through to Duncan right away...