The cursor won't target the staircase.
Added after 1 hour 41 minutes
Oh, I ended up with dragons. I don't understand why there isn't a normal staircase there (I moved to a prohibited sign). You can reach the "enemy dragons" through astral gates. Also, the portal is irrelevant if you have the "Portal to the City" spell.
I completed the map, playing against the computer on veteran difficulty (very easy): I completed the dragon quests, and the computer became the enemy – everything worked. Thank you.