Walkthrough tips, Discussion of Campaigns [Heroes 5]

Yes, it's the third mission for Inferno (campaign 2). There's one Inferno Castle and three Elven Castles.
First, I built up my army (over a week) and took the lower castle. I developed the castle as much as I could, captured all the mines in the area. Then, Gilraen arrived with 4 dragons, twenty druids, and a whole bunch of other units. He defeated my second hero and started attacking me.
I retreated to attack the second castle (top left). I captured it. I held it while Gilraen ran around the first castle, and another hero supplied me with creatures from the second castle. He defeated the second hero again (Gilraen had 26 dragons, 317 dryads, 174 archmages, etc.) and went after the first hero again (a week hadn't passed yet). He caught up, and by then, he had 30 dragons. Where did they come from? No one had visited him, and the military outpost only allows you to hire ents. Are they growing out of the ground?