At 24" I wasn't all that concerned. It's not really much higher than most couches or beds that they jump on / off.
Of course the floor below is a big factor. If it's slick, then I'd be concerned or put some carpet runners down.
I did find it a comfortable height for including the humans behind kneeling in a top 1/3 portrait. In my own space, I would need something more like 18"or even 12" because of low ceilings.