Well first off welcome to Wikipedia As far as I know thats under the additional Expert rules for more advanced play I just looked over our section that deals with it and althought we have it already it isnt too terribly clear about the fortification rule you were talking about I plan on clearing it up in the next day or two but for now I was just wondering in the  rulebook does it only mention altering the value of risk card sets the fortification rule and limiting armies per territory or are any other ones included Thanks