Between the hammer and the anvil.

I killed the demons, captured their cities, explored the map as much as possible, but there are no cutscenes or messages, and I can't approach the gnome – what should I do next? When I capture the second city of the Dark Elves, the quest is not marked as completed.

I checked again: first, I captured the additional city of the Elves, then the right city of the Demons, and then the left one. Both quests were completed, and the passage behind the gnome's garrison was opened. Then I did the same, but captured the cities of the Demons in a different order – and it still works! What sequence of actions did you perform? :)

Ment, JonnyP, have you encountered this problem?