http://grooveducation.blogspot.de/2012/01/how-to-p lay-stronghold-crusader.html

1. make sure your computers are connected each other, via LAN cable (RJ45). it doesn't need internet connection at all.
2. open stronghold crusader, choose multiplayer
3. choose internet direct play
4. if you are the host, don't fill anything. If you join, fill the host's IP address which appear in host's computer
5. then click join, the host's name will appear
6. choose that name, click join.