July 1, 2025 8:45 PM (GMT+2) It requires a key, however I’m unsure if it’s a special key or any key will do (I had no keys when I reached the door).

Note that to access this door, I had to completely drain the fountain using the Pump room.

July 9, 2025 1:52 AM (GMT+2) This mystery as it was worded is technically solved, because I managed to enter the basement from another place (through the Tomb). However, I still do not in fact know how to open the door. Since I suspect something like this might happen with other mysteries, I’ll create a new status “semi-solved”.

July 16, 2025 1:55 AM (GMT+2) I got the basement key from the Antechamber and permanently unlocked the door. So this mystery is now fully solved.