Windows 7 or newer (64-bit), macOS 10.10+, or Ubuntu 14.04+.
Right-click the game, select , then Updates to ensure "Always keep this game updated" is selected. Download Kingdoms and Castles v120r3
If you need a specific older version (like v120r3) and it is no longer the "Live" version, check the tab under Properties to see if the developers have made that specific build available via a drop-down menu. Windows 7 or newer (64-bit), macOS 10