"Ashes' Will" - "The Last Soul" [2nd campaign, 1st mission]

I have a problem.
I can't enter the portal. It's the very first portal from the captured castle of Endril-Sadug.
The hero just stands in the portal, but can't move.
How do I get past this?

Added 1 minute ago

how to upgrade units without a Sorceress? I don't want to go back to her again, I've already cleared out the dragons there.

Where you captured the first castle, there is a Fortress on a hill.