Subway Line 2 to Dongzhimen Station. Get out from Exit B. Then you will see the entrance to Dongzhimen airport express station and the Dongzhimen bus stop. Then walk forward along Dongzhimen Waidajie till you find Dongzhimen Waixiejie on your left hand. Turn into the street and walk a short while to find the Dongzhimen Traffic Hub. There is 936 bus to Mutianyu. Remember, there are only two direct 936 buses a day to Mutianyu Great Wall in early morning.