ToME: the Tales of Maj'Eyal

Either by the portal on the world map (portal symbol has disappeared from it), or by using the Rod of Recall (which has been upgraded on previous visit). Using my modified RoR now takes me to the vestibule area where I fought the Weirdling Beast on my first visit, but, no creature is there now, and the door into the old fortress is closed.
The Quest Log shows what I have done so far with this character (a human higher Alchemist), and the Fortress Energy counter still increases as I transmogrify inventory items in both the Transmogrification Chest and from my general Inventory.

How can I get back into the Fortress now?

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" It is said that Crazy Horse had the power to dream himself into the real world...
...and leave the illusion behind! "