To get to route 120, you have to get through Fortree city. Climb up a ladder (I think there are 2 but I'm pretty sure they both take you to route 120. If not, just try the other ladder) and walk on the tree houses and rope bridges for as long as they will take you. At the end of the chain of tree houses, there is another ladder, so climb down it. Depending on where you get down from the ladder at, there might be a curve that you have to go around, but you should see the entrance to route 120 from there.