when the Targaryens last ruled Westeros. And they didn't work. They couldn't penetrate the dragon's hide and were only effective if they could manage a direct shot into their eyes.
So, it's kinda interesting to me that the show took a different approach. And that they hyped the dragons up to be this unstoppable force, before killing two of them very easily (imo), and the having them return to the unstoppable force again.