Hello!

I'm at Durlag's tower and I can't remember how to get to the roof. How do you get there? I went to the top floor but I can't find the exit.

You don't get to it from the top floor. You get to it from the second (above ground floor) upper level, through a door that leads to...
****SPOILER****
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... a basilisk ambush. Then stairs up to the roof.

Thanks!