Walder Frey is the Lord of the Crossing and oversees the river crossing at the Twins, two identical castles on either side of the Green Fork River. and his army need to cross the river to gain a strategic advantage over the Lannisters after the two families go to war. negotiates with Walder Frey to ensure that Robb and his army can cross. In that negotiation, Catelyn promises that Robb and will both marry children of Walder Frey.