Did you free them before you gathered the garrison? I only managed to reach Direll at the end of the mission.

Yes. I captured the enemy castle, then freed the hero in the dungeon, and then the second one. During that time, I didn't have enough time to gather the required number of dragons in the castle. It just happened by chance. An un-upgraded hero in the castle suffered heavy losses at the beginning of the mission. Also, Faydaen took all the dragons with him on the first day. And Faydaen summoned reinforcements from the main castle several times, and the auxiliary heroes transported troops from the second castle there. I don't remember how many auxiliary heroes there were.