Locate the secret entrance using the video data | Main story Batman: AC Guide

Jump back down and head north, towards the big locked gate. Turn left and you should find yourself beside the wall seen on in the video #1 #2.

Approach the wall and examine the fragment below the CANCELLED sign #1. Press A to interact with the secret panel and get ready to eliminate one more assassin. Wait for the right moment #2 and press Y. Batman will use her to unlock the secret passage, of course you should use it.