Today I traveled by car to visit this site.  Many others have described the fact that it's surrounded by structures that you actually don't see inscribed area from a distance.  They also did an accurate evaluation of the site which I used to orient myself. I took the advice and put Plaza Mayor and then added parking in my GPS and found the parking lot Obispo Galarza ( 2 Euro for 3 hours).  As you exit the garage, Head to the right.  There is a walkway with an escalator on your left.  Head down that way and bear right at the bottom.  The will be a sign for Historic Center in Spanish taking you off to the left.  At the end you are at Plaza Mayor.  Across the Plaza looking to the left you will see the Tourist Center.  They will give you a nice map and I told them I was here for the UNESCO sites and the lady gave me a brochure that is a publicity piece but has more of the history.  Enjoy.