Earlier I posed the question what would happen if my Kensai/Mage accepted Nalia's offer and took charge of the keep... would I still be able to acquire the Sphere as a stronghold?

No one provided a definitive answer, but the general concensus was that the Sphere was the cooler stronghold.

So I ditched nalia and completed the sphere quest. I went back to Nalia and her offer was still open to me, so now I have both the Sphere and the Castle.

I just thought I'd post this for anyone who's interested.