I am planning on visiting the Great Wall in a few weeks. Is the Wall still very accessible during the Winter?

The Wall is still accessible in winter, but the buses may not run frequently as in other time.
For instance, the direct bus for Mutianyun will stop and visitors need to transfer to reach it. But if you only want to visit Badaling section, no such effect.
The tour operated by tour agency is not affected as I know, but it is still suggested to consult with certain tour agency for confirming.
It is not more difficult to access and climb if there is no snow. If it snow, you may need to take care when climbing. It is suggested to wear warm clothes if you plan to climb.