Exit building through door. As you exit the building, a group of Locust will attack. Kill them. This area wraps around to the right. Head that way killing Locust as you go.
At the right side will be steps up. Climb up the stairs.
At the top of the stairs will be a wall. Go around the left side and follow walkway to the door. Enter the door. Go to the right. At turn go left and up the stairs. When you reach the top of stairs go left.
Follow this catwalk around the left side.
At the door a Locust will attack. Kill the Locust. Go through the door and head left down the stairs. A couple wretches will attack on the stairs. Go straight ahead and then to the right. Turn left to face some Locust.
After the battle, go around the rock and down the right path. Down this path will be a troika gun emplacement. Kill the emplacement and the other locust that come out of an emergence hole.
Go straight down this path until you can go left. Go left down to the right turn. Go through the archway and around to the right.
Go up the stairs to the left. Walk up to the gate. The gate will be opened after a short conversation.
Enter the gate. Marcus will start walking. Follow the path straight. When you can go left do so. Walk down this way until cut scene begins.