asseco
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No, that's not true. When I captured the first outpost, all the troops in my army, except for eight angels, were already upgraded. When I returned to the castle, I upgraded them too, but that took extra time. To clarify: I didn't build griffins, I left the armored troops in the castle, and I didn't upgrade the barracks.
I don't think that would have made the game significantly easier.
If there are no mines, the Fortress is very useful; you can put all your resources into the Magic Tower. And fighting with upgraded troops in the Fortress is much easier. Actually, I didn't understand: why do you encounter a Fortress in the Dungeon right away (when it's no longer needed), but there isn't one above?