If you want to play the Adventure-Horror-Game Downfall on Linux, you'll currently also need The Cat Lady, because it needs some libraries from this game to start.

Tested on Debian Jessie 64bit:

1) Install Downfall and The Cat Lady via Steam

2) Open the the data-dir of both install-folders, eg.

/your/path/SteamLibrary/steamapps/common/Downfall/data

and

/your/path/SteamLibrary/steamapps/common/TheCatLady/data

in your favorite filebrowser

3) rename the libs32 and libs64-directories of Downfall to eg. libs32.old and libs64.old

4) copy over the complete libs32 and libs64-folders from The Cat Lady to Downfall

5) if you want to double scale the screen-size, open the file /your/path/steamapps/common/Downfall/Downfall in a text-editor and append "--gfxfilter hq2x" on the execution line, like:

... "$SCRIPTPATH/data/ags64" "$@" "$SCRIPTPATH/data/" --gfxfilter hq2x

There are 2 lines like that, depending if you run the game in 32 or 64 bits.

Source: Steam Forum