When an opponent casts a spell, if this permanent is an enchantment, it becomes a 4/4 Serpent creature with "This creature can't attack unless defending player controls an Island."
Cycling

(

, Discard this card: Draw a card.)
It triggers when the spell is cast, which means it becomes a creature before that spell resolves.
It changes into a creature even if the spell is countered.
When it turns into a creature, it is no longer an enchantment.
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.