Opal Caryatid
Enchantment
When an opponent casts a creature spell, if Opal Caryatid is an enchantment, Opal Caryatid becomes a 2/2 Soldier creature.
Illustrated by
Urza's Saga#24CommonEnglishLegality
Rulings
When it turns into a creature, it is no longer an enchantment.
It changes into a creature even if the spell is countered.
It triggers when the spell is cast, which means it becomes a creature before that spell resolves.
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.