in simple steps :

1) download and instal (buy from steam) on single workstation Stronghold Crusader HD
2) JUST ONE COPY OF THE GAME NEED ! - Even if you bought it from steam 🙂
2) If you using windows 10 go to 2.1
2.1) GO TO :
D:SteamLibrarysteamappscommonStronghold Crusader ExtremeStronghold Crusader.exe
RMB on Stronghold Crusader and select check DPI and set windows XP (sp3)
2.2) on second workstation go to the same spot and set check DPI and select windows Vista
2.3) This process will cancel the problem with (the game is already running ) and solve the problem with No mouse in the game (thanks for TIP from support ibik 🙂 )
4) Work on full screen/fake full screen - no extra settings need

Ready for your Crusader?
If yes you can easy go to multiplayer and select create (dont do nothink with IP and port)
and second player just click Join and wait around 10 sec.. and the game will appear 🙂
ENJOY!