The gravity well feature from the Graviturgy Magic arcane tradition triggers "whenever you cast a spell on a creature", and has an effect "if the target is willing to move, the spell hits it with an attack, or it fails a saving throw against the spell." It’s not clear to me what kinds of spells can actually trigger this feature.
Based on the singular phrasing of "on a creature" and the later use of the word target, I would normally read this as only applying to spells that explicitly target creatures (and perhaps only those with a single target, in contrast to the phrasing of the Sculpt Spells feature of the evocation wizard, for example). However, spells that target a location can use the word target in a different sense (see What counts as a target for a spell?).
What’s actually going on here? Does this feature only work on targeted spells, or does it function for area of effect spells as well, or is this just an ambiguous phrasing that will rely on DM’s ruling?