Ahhhh thank you! (not so silent scream of frustration and gratitude) I had somehow managed to explore almost all of the map, except for where his first tower was. Sheesh. Walked in, did the things, go back to the second tower - boom, open. Thank you for your guidance, O Knower of Spiderweb Things! May your battles be victorious and your snackrifices tasty.
I have accepted Bevan's quest, spoken to Abby and found Sixus' tower, but I can't actually get inside because the main gate is shut! What am I missing? I'm spinning in circles trying to figure this out.