Opal Caryatid

It triggers when the spell is cast, which means it becomes a creature before that spell resolves.

When it turns into a creature, it is no longer an enchantment.

It changes into a creature even if the spell is countered.

A "creature spell" is any spell with the type Creature, even if it has other types such as Artifact or Enchantment. Older cards of type Summon are also Creature spells.

A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.