To fall down is to drop from standing to the ground. Fall down the steps, watch him fall down, the wall will fall down.
It works for people and for buildings alike, and the down adds finality rather than direction. Figuratively an argument can fall down at a particular point, which means it fails there.
1phrasefall downTo drop from standing to the ground.
“Toddlers fall down twenty times an hour and mind none of it.”
fall down is a phrase.
Also accepted in games: fall over, fall
Practised in Phrasal Verbs