“Ashes’ Will” – “A Sad Journey” [Campaign 2, Mission 2]

Hey everyone! Does anyone know where to find the secret entrance to the red castle in the Tribes of the East campaign in Heroes of Might and Magic V? And what is the red key for? (P.S. I'm not blonde=))

In the center of the map, there is an entrance to a dungeon. Go there, and you'll need the key. After that, a quest will appear asking you to find the entrance to the dungeon. Go to the dungeon near the last Academy castle, and that will be the path to the last red castle.