Ah, but none of these possibilities PRECLUDE the fortress being visited in the future, which by definition makes the fortress not irrelevant.
This is actually a clue concerning the debate about Durkon's desires. If he had not wanted to remain a vampire, he could have let himself be dusted. Instead he fought to get the staff back. This at least suggests, if not definitely proves, that the entity in side Durkon, at this moment, wants to continue existing in its current state.