I have a 10 hr layover in Beijing. Would I have time to visit the Great Wall? Do I need a VISA?

If you are goring to have a transit from international to internation, with a transit permit alone (due to 72h free-visa policy), you are allowed to get out of the airport for visit. In this case, no visa is needed. To confirm whether you are qualified for the 72h free-visa policy, read the infor on this inlk:
https://www.travelchinaguide.com/embassy/visa/free-72hour/

Generally, if you take taxis or driver to come and back, the hours shall be enough to have a short trip at the wall.