I just saw The Force Awakens and the Starkiller weapon reminded me of the novel The Crystal Star. This book was my first encounter with world-craft class starships which I thought were a very cool idea. I assume that the writers took inspiration from this novel as they did for many other parts of the movie from other books.
Was the Starkiller a World-Craft or was it a planet that was modified?
According to the novelization, it was a naturally eroded planet/area which had been further excavated.
The fleet of Star Destroyers stood off the white world. Spectacular and isolated, with a mean surface temperature varying from merely cold to permanently arctic, the planet had been altered: its mountains tunneled into, its glaciers hacked, and its valleys modified until it no longer resembled its original naturally eroded form. Those who had remade it had renamed it.
Starkiller Base.
Hollowed out of one snow-covered mountain was a central control facility. At its heart was a great assembly chamber that held hundreds of workstations and their attendant seats.
