I worked the stone, and we jumped into the house which Erik had built himself in the double case of the foundation-walls of the Opera.

And this was the easiest thing in the world for him to do, because Erik was one of the chief contractors under Philippe Garnier, the architect of the Opera, and continued to work by himself when the works were officially suspended, during the war, the siege of Paris and the Commune.
I knew my Erik too well to feel at all comfortable on jumping into his house.

I knew what he had made of a certain palace at Mazenderan.
From being the most honest building conceivable, he soon turned it into a house of the very devil, where you could not utter a word but it was overheard or repeated by an echo.

With his trap-doors the monster was responsible for endless tragedies of all kinds.

He hit upon astonishing inventions.