Ranger – “Emerald” [5th campaign, 2nd mission]

You're not reading carefully. The Fortress guards the entrance to the Dungeon. On the first week, you don't even need to bother with them...

Actually, thanks for the castle placement. I was posting it out of spite because of the Ents on the first week, in a hurry, and used to the advice "on the first day, buy both heroes and build...", when you only have five thousand (Demon-3 campaign, CRPG playthrough).
After the rearrangement (or castle placement), it became easier - and I passed it right away.
All I had to do was freeze the hero with the Elves on the first week.