How do I complete the final mission playing as Roland?

That's strange, I've never had any problems there. I always took the crown after completing both storylines. Then, in the final, I gathered a few resources, checked the castles for the presence of a "portal," and that was it. It's even easier with Roland, as you don't need to travel a long path to reach Archibald; you can simply jump across the gaps (found by trial and error) on the left side of the map.