Playing Stronghold Crusader with a few friends is actually really easy, you just need to forward a few ports on your router and give them your IP address.
The ports you need to forward on your router are:
2300 to 2400 for TCP and UDP
47624 for TCP and UDP
6073 for UDP
After that, just select Internet TCP/IP Connection For DirectPlay and host your game. Your friends can follow the same step but type in your IP address when prompted and join you at the round table.