The official website for The Settlers of Catan

Question about Barbarians & Traders Dogma - Can I place a Dogma on a Building that already carries a Witch?

Answer

No. You could have played the Witch only if you already owned a Citadel - but if you own a Citadel, you no longer can play a Dogma.

Note: The Citadels and Witches belong to the theme set "Wizards & Dragons", which is part of the Catan Card Game – Expansion Set.