Here's the trees:
Weirwood - Game of Thrones Wiki - Wikia
Here's Bran:
Bran Stark - Game of Thrones Wiki - Wikia
You're right, the walkers and dead need to overcome some magic at the wall before heading south.
Bran just needs to be able to touch a heart tree.. a weirwood with a face carved into it to enter his vision of the past... as they escaped the one underground tree area, he remained in the past, unconscious, while that gal carried him out, he was also in the present some how controlling Hodor... that's what messed Hodor up for life... it attached the young Hodor of the past with the old Hodor of his future until the moment he died. "Hold the door" ... "Hodor" Bran entered the past again at the heart tree near the wall to see shortly after Jon was born.