RTF is a global modification that replaces all towns in the game.

As neutral units, but with a stylistic connection to a specific castle?

They could appear as neutral units with a stylistic connection to the Haven faction.
In the Unicorn Empire, there are different duchies, including the Hound Duchy.
So, I think we might be able to play as them in campaigns/scenarios.

Added after 2 minutes
They could also appear as:
Griffins (Griffin Duchy)
Wolves (Wolf Duchy)