Check if the "Castle 3" base file has been updated. Often, an "HD" add-on is designed for version 1.2 of a map, but the user is running version 1.0. Ensure the base and the HD overlay versions match.
Most "fixes" for HD environmental mods fall into three categories: candid+hd+castle+3+fix
If the game crashes specifically when you approach the castle, it’s a rendering bottleneck. Open your configuration files (.ini) and ensure that your Texture Memory Budget is set to at least 4GB (if your hardware supports it). Check if the "Castle 3" base file has been updated