With the recent release of Atomfall, there are a lot of cool places to explore, just like the one you can find in this guide below. Let me show you a quick step-by-step process to getting the castle key and finding the lock/door location inside the Castle Ruins in Atomfall.
Where to Find Castle Ruins
The Castle Ruins area can be found in this part of the map. The following coordinates are 19.6E, 83.7N.

If you end up walking in front of this forgotten castle, then you know you made it to the right place!

Castle Key Location
Once you enter, walk along the hallway until you find an entrance on the left. This will lead you to another corridor. Keep following the path until you stumble on this long table with candles on top. You will easily find the key there.

Lock/Door Location
Using the key location as a starting point, simply follow the path towards the right until you find yourself in a dining hall. You will see tables dressed in white cloth, filled with spoiled and untouched food. Walk to the front, right next to the throne. You will find a gold-plated lock on the wall to its right side. By using the Castle Key you just equipped, you can open the hidden door behind the fireplace.

I hope this guide was able to help you easily locate both the castle key and lock/door location inside the Castle Ruins in Atomfall. Now, what actually happens beyond the door is up for you to find out. No spoilers! If you’re a new player and want to research more about the game, we both have guides on how to barter and upgrade guns in Atomfall! Check ’em out.