How do you get the Mage Stronghold? Where do you go and who do you talk to?

*SPOILER*





talk to delon in the government district (outside the main building) to get the umar hills quest.

then go to umar hills and get valygar, agree to help him with the sphere quest and then go to the slums

The sphere is in the top left hand corner go in and finish the quest

it all yours now...............

(sorry if's that a bit vague)

If your evil, his dead body will do the trick.

SPOILER
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if you mean the place where the cowled wizards took imoen and irenicus, you have to get the 15000 for gaelen or bohdi and do some quests for them and they'll take you there. if that's not what you mean i don't know where it is.