Climb forward and up onto the catwalk. You are headed toward Park Row. You'll find a few guys on your destination. You have an armored guard, a shielded guard, and gun guard. Focus on the guy with the gun, then the guy with the shield. The guy with the armor is easily countered whenever he strikes. Once you've dealt with them, go into the sewer. Run along the sewer, into the confiscated goods vault. Go into the next room and after she talks to herself a little, jump onto the ceiling and head into the next room. You want to find three guards and pick their pockets (not take them down).
Head back to the control room and use the computer to unlock the door. Head back towards the vault door and clear the area. Just isolate your enemies and pick them off. Be sure to use the shortcuts in the stacks where boxes are missing.
Head into the vault door and check the plant and the briefcases. You'll have to fight your way out. One enemy has armor and the other has a shield but otherwise they aren't anything special. There are six in total. Just make sure to use the space you have in here to your advantage.
Now grab the briefcases and head out. When you're almost out, you'll have to decide between saving Batman and walking away with the briefcases. You get to choose. While there is a wrong choice, it is an interesting choice to make at least once and won't really set you back any effort. When you're ready, go save Batman. Crawl through the vent and you'll automatically make your way to the Steel Mill.
