Sometimes, players will share their maps through means other than the Workshop. We strongly recommend you use an anti-virus to scan files you download before opening them and only download files from trusted sources.
Once you've downloaded the file, if it is compressed (zipped), you will need to extract (unzip) it. Most operating systems will allow you to extract compressed files by right-clicking them and selecting the appropriate option.
You should now have a folder that contains files such as "Factions.json", "gameobjects.json", "Heightmap.png", "LuaMakersCount.json", "meta.json", etc. This folder is the map's folder.
You will need to move that folder and all its contents in the directory located at:
NOTE: If you have the Steam version of the game, you can locate your "Castle Story" directory by navigating to Steam/SteamApps/Common/Castle Story
Once that's done, you will be able to load a new game using that newly installed, player-made map. Enjoy!