I walked on the Great Wall in December 2010. Beijing's bitter winter made it a memorably cold WHS visit - indeed it is joint coldest in my mind with Denmark's Kronborg Castle, which I visited in a more recent winter. The thing that surprised me the most was just how steep the wall is. I checked-in at the tourist-heavy entrance at Badaling, though in the winter it wasn't overly busy at all. It’s about 90 minutes drive outside of Beijing, and a real contrast to the densely-packed, smog-choked metropolis.