Valley Of Tranquility

Valley Of Tranquility enters the battlefield tapped.
When Valley Of Tranquility enters the battlefield each player gains 3 life.
Whenever you gain life add {g} to your mana pool.
{t}: Add {g} to your mana pool
{g}, {t}: Tap target creature you control add {g} to your mana pool equal to half it's toughness rounded up, only spend this mana to cast creature spells.