Where is the key to Saarthal in Skyrim?
To open the door to Saarthal go to College of Winterhold (that’s where they train mages) and start your mage training. Your second task in the mage training is to meet Tolfdir (your instructor) at Saarthal. It takes him 3 hours or more to get there so if you beat him to it, just wait.
He will unlock the door and you will be doing 2 quests here Under Saarthal for your mage training and Forbidden Legend.
This question comes up either because you stumbled onto Saarthal while exploring, or you were doing the “Forbidden Legend” quest, and trying to put together the Gauldur Amulet, by collecting the 3 amulet fragments in Saarthal, Geirmund’s Hall and Folgunthur. If you get to Saarthal before you joined the Mages Collage, you find a locked door.