Spec Ops: The Line - Chapter 8: The Gate

After, you'll go through a large door and into a rather nice building. Move through this building and you'll find "the Gate". After the cutscene, you'll take control of a mortar. Use the screen to aim. Move the screen with either analog stick, zoom with the left trigger and shoot with the right. You always want to go for targets with the red box around them. These targets are a primary threat to you. Feel free to destoy any other enemies or vehicles in between these targets though. Soon, you'll be done with this and have to move out.

Rappel down the lines to your left and then move through the area you just destroyed. You won't actually need to shoot anything until after Walker gets all pissed off.

You'll come out of the far side of an alley and it will open to this big sand hill. You'll have to fight your way UP the hill and Adams will be sure to point this out. This isn't an easy battle. You'll want to move up but don't go past the jeep on the right side. Instead, just left of the jeep will be an unmarked column (not the one with the 33rd logo on it). Stay here and thin the enemy a little bit. You will move up to the column with the 33rd logo on it but you need to make sure the flanks are safe first. Move up when you can.

From this position, you should be able to clear almost the entire area. Make sure it's safe before you make your way up to the entrance of the building. Approach the top and a group of men will be standing in the open in front of you. Kill them and make your way in. I found a couple of enemies coming up the stairs to the right when I entered but I'm not sure that happens every time. Kill them but stay on the top floor.

From here, try to kill a few enemies, particularly at the base of the stairs. Get to the bottom of the stairs and immediately take cover. You want to stay here for the rest of the barrel. Be sure to watch the large flight of stairs ahead and to the right. Not many enemies come down this way but you don't want to be caught by surprise either. Clear the room of enemies and then climb up those stairs.