As a former student of the local university, I had been many times to the cathedral but being up on the roof showed me a lot of things I didn't know about the cathedral.
They will take you to the cathedral stone roofs (don't wear high heels!), where you get great views of the old city.
www.santiagoturismo.com/
€10 for 30-45 minutes, everyday 10am to 2 pm, 4pm-8pm. Booking recommended (especially in summer), entry via Pazo de Xelmirez, Praza do Obraodiro.
Updates
Being Galician myself, I did this tour mainly for my boyfried and a guest who was visiting at the time. You'll love the views and the stories. I agree with "gantius", no heels!
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