Two sisters.

If there had been a Fortress there, the game would have been five times easier.

Nah... that's not true. By the time I reached the first outpost, all the troops in my army, except for eight Angels, had been upgraded. When I returned to the Castle, I upgraded them too, but that was just extra time. Let me explain: I didn't build Griffins, I left the Armored Knights in the Castle, and I didn't upgrade the Barracks.
I don't think that would have made the game significantly easier.