My friend and I noticed that there seems to be an inaccessible room in the Tavern with something behind it. We saw that there was a Treasure Chest seemingly floating somewhere off the map.
In addition, if you walk up to the second floor of the Tavern and position your camera through the wall just right, you can highlight something called the Defender's Forge.
Confirmation of the speculation was confirmed in the notes for the 7.07 patch, which had a note that said the following:
Moved bonus-item crates back into the main section of the Halloween Tavern, out from the Secret Room.
What is this Secret Room? How do I get there? Do I need to beat the campaign first or something else?