Make your way out the house onto the balcony and you will be able to jump up to another balcony with your hook and down onto another terrace where you’ll get shot at from a police gondola.
Once you get down onto the terrace, make your way into the house. In there you will find a couple by a printing press, they are friendly. Go past them and out the doors on your left where you will find considerable enemy. Turn left when you get out the house and go straight and you will see some steps up to an darker imposing building with a big eye carved on the top. I think this is called the Order of the Raven or something along those lines.
There will be some sort of ritual going on in there, up stairs on a stage. Eliminate all the enemies and the key can be found on the main stage table.