You typically need to own the original Party Hard game and, in some cases, beat the first level of the main game to unlock the Dark Castle stage.
If you own the base game on Steam, the DLC is often added to your account automatically or can be toggled in your library's DLC list. download-party-hard-dark-castle-the-games-download-exe
Rather than searching for a standalone executable ( .exe ), the safest and most standard way to access this content is through the base game's official store pages. You typically need to own the original Party
It was developed by Pinokl Games and published by tinyBuild. How to Access the DLC It was developed by Pinokl Games and published by tinyBuild
It adds a new "Van Helsing" themed playable character, a new level set in a haunted castle, and a trusty dog companion.
You can find the DLC directly on the Party Hard: Dark Castle Steam page.