The only class that was able to make it to the top of a wall was Thief with shortbow which I'm certain was unintentional. The game is pretty determined to prevent players from using abilities to cross gaps or reach places that you are unable to run/jump to. With certain leap abilities, like the Guardian's greatsword ability you can cross small gaps that are not jumpable, however.
As for knocking people off of walls that is definitely possible and I have done it.
However, just because they're dead on the ground outside of the keep doesn't mean they're out of the fight. Most of my time spent defending keeps on my Guardian involved me jumping down with another group of melee fighters, skirmishing the enemy quick, reviving some people, then all falling back into the keep. Heal up, rinse, repeat.