From that ledge and whilst facing the crane, drop off the ledge and onto the crate below to the right (1). Turn so that the crane is on your left and drop backwards to land two blocks below (2,3). Turn left to face the crane and notice the metal ledge below attached to the body of the crane (right hand pic). You need to reach that ledge, but you need to set up your jump.

Facing the crane, walk to the edge of the block and stand in the front right corner (exactly next to the higher block). Turn a bit right and use the Look button to check if you are angled to the ledge. Now take a standing jump without pressing Action to land on the metal ledge. From there take a running jump to grab the roof with the barbwires. Refer to the Close to the crane part almost at the end of the walkthrough on how you can proceed.

From the starting position, turn left and take a look below at the opposite buliding. Notice the Small Medi Pack on a slope attached to the building (spot A). You can take a running jump to this slope (losing 50% of health), grab the crevice above, traverse to the right and drop to the ledge at the end (spot B). Or, you can take a running jump derictly to spot B. Then climb to the ledge above, vault onto the slope (spot C) and go to the left. Follow the path and enter the passage on the right to reach a ledge above the area with the scaffolding.

Do not proceed further and do not jump the gap. Turn right and take a look at the wall. There is a part that is lower from the rest of the wall. Stand against the lower part and jump up to grab it. Do not pull up yet. Instead, shimmy to the left until you cannot shimmy anymore. There is a flat roof to left of the slide, where you can jump onto.

Pull up and immediately (before even sliding) press Jump along with the left direction key to land on the flat roof.

From there, drop to the lower roof and head to other side. You can explore the area around, but be careful of the barbwires. Note that you cannot return back and if you want to finish the level, simply head to the end of the walkway beneath the Cathedral ledge (where there are no railings).