Worshipper – “Conquest” [Campaign 2, Mission 3]

Thanks to everyone for the advice; here's how I completed this mission (I'll try not to forget anything):
I chose Fire Demons as a bonus. I waited for two weeks until the start. I defeated Gilraen (GI). Using mages and elementals (only 64 demons left), I miraculously captured the northern (2) Castle. All creature dwellings were built there, and half were upgraded, but there were no Druids. I built Druids and bought 4 of them. I hired another hero and led Talaran (Tara), who was hanging around nearby with a decent army, away from the Castle. The next day, I bought an increase and ran towards the northeastern (3) Castle. GI appeared behind me, but I had a significant lead. I captured the fully built Castle 3, bought an increase, and continued to run away from GI. GI didn't try to recapture the Castle!
Surprisingly, I now had a hero in Castle 3 and captured the mines. I went back through the middle to Castle 2 and then up to Castle 3 again. If GI was far away, I captured the mines, gained experience, and visited the upgrade locations. On the third round, I reached level 20 and learned the Portal spell to the city. And just in time. Tara recaptured Castle 3, and GI blocked my path to Castle 2. I used the Portal to teleport to my home city. I took the demons and left the elves on the border. I made another half-round, but not through the cities, but simply across the map towards the Druids. I defeated them. I used the Portal to teleport to the city. I defeated Tara in Castle 1. I hired him in the tavern on the next turn. I gave him the elves. GI chased Agrael, and Agrael led GI to Castle 2. Meanwhile, my Tara captured Castle 3. GI entered Castle 2 and never left. At all. I calmly cleared the map and gathered an army (GI ran into my city with 60 dragons, 230 ents, etc.), and then it was just a matter of technique