Does a cantrip only count as “cast a spell” then?

Does a cantrip only count as “cast a spell” then? A caster can’t use a cantrip on an opportunity attack?

If you try to design monsters to be encounters all on their own, you’re limiting their versatility

No matter what I’m designing, I try to look at it from every point of view

If my temporary HP are 10 and I take 30 damage, what is the DC of my Concentration Check?