Up or down? Go to the parking lot at the entrance of the Great Wall Museum. There are tourist buses and high-speed rail stations. It is very convenient to go back to the city. If you want to take 877, it will be far away. You have to detour to Qianshan
The ferry is a shuttle bus, instead of going directly to the ticket gate, you need to take the ferry to get off and then transfer to the city. If you take the through train, you don't have to worry about it.