6 decades books

Humanity was fighting mysterious monsters called The Shadows for ages, and huge cities known as Havens were built slowly over the centuries. Now the Industrial Revolution, fueled by natural sciences and alchemy of silver, raises mighty towers of concrete and metal and sending airships to conquer the skies. Promise of freedom from The Shadows never being so tangible. Four young girls hailing from very different walks of life arrive in Elysium, “Best Haven in the World”, to become Hunters – the best defense humanity ever had against The Shadows. Will joys of friendship await them here? Or will strength of their personalities only tear them apart?