The Sionis Steel Mill is an integral part of the main storyline of Batman: Arkham City. It serves as a hideout for the Joker and then for Harley Quinn in her Revenge DLC. You will have to find ways to get inside several times over the course of your progression, which can prove to be a bit confusing due to the nature of the map and level designs.
Steel Mill location in Batman: Arkham City
The Sionis Steel Mill can be found directly below the Industrial district in the southwestern corner of the map. It is one of the few locations in Batman: Arkham City that you might find confusing to both explore and enter.
Whether you want to complete your story missions, get the Freeze Cluster Grenade upgrade, or collect Riddler Trophies and other collectibles, you must find a different way to enter the Steel Mill depending on where you are in the main storyline.
How to get into the Steel Mill in the main storyline
The first time you enter the Steel Mill will be through the big chimney with the skulls. This will be during the early parts of the main storyline when you have to track down the Joker’s radio signal to find his hideout.
Reach the high tower with lights (circled in the image above) after which Alfred will tell you that you can enter the Steel Mill through one of the chimneys in Batman: Arkham City. Use your Grapple Gun on the edge of the chimney to trigger a cutscene in which Batman glides down the Waste Exchange.
How to get back into the Steel Mill as Batman
You will need to get back into the Steel Mill a second time to advance the main storyline. This will be during the latter sections of the game after you have rescued Vicki Vale from a helicopter crash caused by the Joker.
You will not be able to use the chimney entrance the second time. There is a second entrance in the south of the Steel Mill that leads you to the Cooling Tunnel B. Oracle will mark this entrance on your map as shown in the image below.
You need to defeat Mr. Freeze before getting back into the Steel Mill.
Something important to note here is that the Steel Mill will be heavily guarded when you are trying to enter it a second time in Batman: Arkham City. There will be two snipers on the drawbridge of the Industrial district that you can avoid by circling around the Steel Mill instead of heading straight south.
We recommend taking out both snipers in the towers as well as other nearby enemies before entering the door.
How to get into the Steel Mill as Catwoman
There are several Catwoman Trophies in the Steel Mill, but you will once again need to find a new way to get back inside since both the chimney and southern entrances will be permanently disabled.
The only way to get into the Steel Mill as Catwoman will be through the huge hole that was created during the Joker boss fight. This will be by the main entrance of the Steel Mill, east of the river, and leads directly into Joker’s Office.
This also means that you need to defeat the Joker first to get into the Steel Mill as Catwoman, or even Batman for that matter in Arkham City.
This entrance is easy to miss because it is covered in flames. You have to jump down to the destroyed walkway and go through the hole.
When you reach the location marked on the map above, climb up to the platform and circle around the destroyed chimney. When you turn back north, or the front of the chimney, you will see flames. Make your way towards it and climb the fence.
Then drop down through the destroyed roof. Turn around and go through the gaping hole to get back inside the Steel Mill.
How to get into the Steel Mill in Harley Quinn’s Revenge DLC
Following the events of the main story campaign, Harley Quinn takes over the Steel Mill as her base of operations to enact revenge on Batman. Her story expansion is split between two sections and both require you to head back to the Steel Mill.
When playing as Robin, you will automatically start inside the Steel Mill, but when the game switches to Batman, you will be in the Amusement Mile and have to find your way back into the Steel Mill. The problem remains the same here. There will be a waypoint marker on your map but it will lead you to just a destroyed chimney. So how do you get into the Steel Mill as Batman?
You have to use the same entrance that you used to enter as Catwoman. Circle around the chimney and drop through the broken, flaming roof. If you are confused, this is the same location where you were at the beginning of Protocol 10.