Also you can always use watch towers to defend the river pass. Then, you should create docks and you can block EASILY the pass from the red guys with scout ships and war galleys. I just converted the yellow guys and then building my civ in that area, creating and converting villagers.
I did it quickly when I tried to convert yellow guys (I tried in the hard way, trying to convert red guys but is impossible, the red is cheater because they can see you even if your priest is out of sight).
