The work of constructing this double case took a whole year.

It was the wall of the first inner case that the Persian struck when speaking to Raoul of the house on the lake.

To any one understanding the architecture of the edifice, the Persian's action would seem to indicate that Erik's mysterious house had been built in the double case, formed of a thick wall constructed as an embankment or dam, then of a brick wall, a tremendous layer of cement and another wall several yards in thickness.
At the Persian's words, Raoul flung himself against the wall and listened eagerly.

But he heard nothing ...

nothing ...