Steelrising

Can be done by playing through Cagliostro’s Secrets DLC, which is started by speaking to Abbe Gregoire in Luxembourg and should be started AFTER completing the Bastille section of the game, the DLC doesn’t become available until around that time.

Note: there are a couple pre-requisites prior to starting this, but we shall skip the parts that require opening two locked doors with 2 different keys in the dlc for finding out the location of the secret area, everything you need to know is down below:

If you’ve played through the DLC questline up to the boss, you would have found the “Cast Iron Key” which cannot really be missed. You want to head to the Bastille map (before or after you defeated the final boss of the DLC, it doesn’t matter, just have the cast iron key with you) where you should spawn outside the garden area. Proceed inside and enter the tunnel drop down, at the other exit, you’ll emerge back into the gardens exit with a pedestal next to it. Proceed forward a little more and you will find yourself on the dried up moat area, more specifically above it on the surrounding bridge of the castle.

Go left from the entry to the moat bridge and eventually you will find a gate that requires unlocking. Proceed through this area and explore a little. The “end” (you can choose to go left or right towards the end, go right) of this area will lead you out into the bottom of the moat, you will see a bridge straight ahead which you can grapple up to using +

Pull the lever that’s off to the right of the grapple point at the bridge and you’ll unlock a shortcut back to the main area. The achievement will then unlock.