A ladder only has one path. There is only one measure for a ladder: how high you are. To reach a step you have to have had climbed the one before. A ladder goes only in two ways - up, or down. Ladders are boring: once you've figured out how to climb a step, you know how to climb the next one. You can't overtake someone on a ladder, going up or down. Once you're on top of a ladder, to climb another ladder, you first have to climb down.
A wall has many paths to climb. You might reach the top or you might not, but wherever you end up, there's always an interesting journey behind you. Climbing walls are like solving puzzles: out of all the possibilities, which one can you take? There are many skills required to climb a wall, climbers will rely on some skills more than others - and that's ok. You can climb the same wall 10 times and still learn something. If you're climbing and you're stuck, you can back down a bit and try a different path. Many people can climb the same wall at the same time, and reach the top. You can move sideways on a wall. Some people stop midway on a wall and just enjoy the view - and that's ok.
The way to the top is not a ladder, it's a wall.
