So... apparently Braccus Rex is a racist against mostly dwarves and doesn't let them reach the Gargoyle portal switch in the labyrinth with the flaming Historian!
I have tested with a Lizard and they can reach it. Now my Count Dwarfcula is stuck in the realm of Orobas for eternity! :P
I've noticed my human female characters cannot access the other portal in the labyrinthe, the one near the ground switch. Same with the chest in the hounds room. Elves and lizards can access them.