[Azaghal]

There is a mod out there that let's you play all strongholds even if you are not the particular class. Since I haven't played the game without the mod for a very long time I am not certain about my memory concerning the non-mod ways: If you start a stronghold you get "blocked" for all other strongholds. So you can only start and finish one. However, the choice which one you start is yours if you are a multiclass character.

IMHO the thief stronghold is rather boring, you have a few simple missions and then you have to return there every week or so to collect the funds and pay Renal's share, otherwise you'll loose the guild. The sphere is not much better, a few simple quests and then you have to tutor a group of mage apprentices. But if you're willing to play risky, you can earn an interesting magical item at least.
IIRC there is guild improvement mod out there. Check the mod thread on top of the page.


SPOILER
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If you want to do the sphere you have to get Valygar from Umar Hills first. Dead or alive.