How did two of them enter Castle Black in season 2?
shareThe wall is just a physical barrier, it can and has been circumvented. Though it is extremely difficult. In the end that is the main purpose of any barrier.
shareBut in Season 6 when Uncle Benjen was talking with Bran and Meera, Benjen mentioned the wall was filled with magic crystals to prevent them from going thru.
shareI only remember the dead man they brought to castle black and turned in the night. Jon burned him.
shareThe dead men had been turned in to zombies before they were brought thru the wall. They shouldn't have been able to be brought thru.
shareThey were dead, then they turned.
shareBut that's how. They were brought. They wouldn't have been able to enter on their own once they turned, but other members of the NW carried them in before they turned.
(Also, that happened in Season 1.)