The earliest you can unlock this trophy is mid way through sequence 2.
You will be asked to help out in a scripted den defense, during the den defense you will be asked to place a leader, then place 3 Crossbow men, then place a barricade, then you will be asked to place another leader and then a Rifleman
THIS IS WHERE YOU STOP.
Once you have placed your first Rifleman simple wait it out, you will collect morale from the assassins leaders continuously and during this time NO enemies will attack, so simply wait it out till you have around 300+ morale ( you get +2 for each leader every 5 seconds)
Once you have 300+ morale start to place lots of Riflemen of rooftops and placing upgraded barricades so the 'onslaught' can continue.
DO NOT PRESS AS THAT IS THE CANNON BUTTONHere's a video by ROCKY v3