About the iron door in the lower left corner of the "Wine Cellar" map

[mike19911209]

How can I open and pass this iron gate? Even after I cleared the level, I still couldn't explore this block.

[TekuStudios]

If you mean the gate on the left, inside the laboratory: It is a secret path. You need to talk to the nest aunt during the objective "The antidote" in the library, who will tell you how to get the inquisitor's ring. After that, investigate a clue to the west of the wine cellar to discover a passage that will lead you to the other side of that gate. It is a shortcut that can only be unlocked during this objective.