If u want to go for wlan, u must place wireless AP at regular intervals along the 10km distance.
To cover ten kilometers, u need this equipment that was shown in DEFCON, the hacker conference. That "wifi pistol" can detect networks 10km away and can login to them as well. But I doubt that it is up for sale.
So ur best bet is to go online and play by either one of u hosting the cs server. I guess u won't feel too much lag this way.