GO FOR IT, NEWBIE!!!

What are these new rules that prohibit upgrading ducks... But can Angels be upgraded?..

You can upgrade everyone, just not in an external upgrade facility, but in the castle.

5.11. Visiting mapmakers and "Hill Fort" is prohibited.

In this case, I knew that the opponent didn't have any secondary Swamp creatures, and it's also unlikely to get 18 upgraded Swamp creatures from Pandora or a quest, so I immediately asked where they came from.