I bought this game specifically to play on LAN with my siblings, as the game page specifically said LAN functionality still works.

I grabbed the game's serial number from the "More" page, but the game says it's invalid. Does anyone know the solution to this?

Use the registry key installation file from the game directory.

I know this is old, but where do i find this file?

It's regs.cmd in your game directory. Run it as administrator.

Can anyone help me step by step how to find the cd serial number please?
Im no computer buff........

1. Go to the folder where you installed Settlers 3
2. Find the file named regs.cmd
3. Right-click and click 'execute as administrator'.
4. A Windows Command screen will open and execute the registering (little black window).
5. You're done, play the game and it should not ask for the key.

It worked it said "regs" right click and it will set the game.