The Opera Hall is an area of the castle that gives you access to the most useful items in Resident Evil Village so far - the Iron Insignia Key, which you receive after solving the piano puzzle.
You may have seen several locked doors in both the castle and the village that requires they key - so having it in your inventory will open up progress across the game significantly.
It's also where you'll get hold of a Castle Map, which is another useful tool to help you explore the area.
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Exploring the Opera Hall and getting the Castle Map
From the Courtyard, once you have the required Key following the Severed Hand encounter, you can access a stairwell west of the Opera Hall. Before you climb the stairs, grab the Castle Map (Annex) from the far wall, which will help flesh out your - you guessed it - map.
Head up the stairs. On the upper floor between the Library and the Opera Hall, you can get the Grand Chambermaid's Notice file.
Go east into Opera Hall. Go straight ahead for the Further Observations file - you need to pick up and inspect the book to do so - and at this point you should hear enemies.
There is one either side of the balcony on this upper floor. Take them both out, then go north, as there's the Flower Swords Ball in a case at the end. Once done, head to the south side and down the stairs.
On the ground floor of the Opera Hall, check the book next to the piano for the Insect Observations Journal file.
Opera Hall piano puzzle in Resident Evil Village explained
Next, head to the front of the piano, and interact with the keys.
This will start a puzzle. Here, you have to play the keys and match the notes. Thankfully, there's no musical skill or memorisation involved; just play keys until the note matches what's on the page, which will see the puzzle shift along to the next note.
Repeat this process - hitting the keys until it matches each time - until you're done; Ethan will then flawlessly play the piece, and the piano will give you the Iron Insignia Key.
Where to go after the Iron Insignia Key
Head through the door from the ground floor of the Opera Hall. Attempting to head upstairs here will see Lady Dimitresu appear and give chase.
Head back down and into the Courtyard, and west into the Dining Room. She - and any enemies you left in the courtyard from earlier - will give chase, so be hasty.
Our recommendation here is to now head north, through the Kitchen and back into the basement, where you can now open the locked room east of the Chamber of Solace - the Special Chambers - using the Iron Insignia Key.
This is optional, but recommended if you want some additional Treasures. If you do, you'll encounter a number of newly spawned enemies on the way.
Otherwise, heading back to the safe room just off the Hall of the Four with the Merchant is a good place to go next - allowing you to sell any new Treasures you've picked up and upgrade your inventory.
Looking for more help? Our Resident Evil Village walkthrough hub offers an abridged guide to completing the story.