When an opponent casts a spell, if Veiled Sentry is an enchantment, Veiled Sentry becomes an Illusion creature with power and toughness each equal to that spell's converted mana cost.
10/4/2004 If the spell is no longer on the stack when the triggered ability resolves, the converted mana cost of the spell is still remembered.
10/4/2004 When it turns into a creature, it is no longer an enchantment.
10/4/2004 It changes into a creature even if the spell is countered.
10/4/2004 It triggers when the spell is cast, which means it becomes a creature before that spell resolves.