The characters correspond to the altitude of a location. Black means they can be accessed from S; red means they can not be accessed. S is one of the ‘a’s at the left rim of the picture, E is a ‘z’ in the middle of the large red ‘flower’. I don’t see any two adjacent characters bridging the black and red areas, do you?
At the end of this it says that you can go down as much as you want so once you get to the black "r" you can drop down to the red "p" and get to the end from there