What do those two lines mean: twelve doors melt cold light, and twenty-three lines move purple emperor?

The streets of Chang 'an are quiet as if immersed in a cold light. The cold air and cold light in front of the twelve gates of Chang 'an were melted by the sound of music, 23 strings were bounced high, and the heartstrings of the gods were attracted by music. Supplement: Purple Emperor is a pun, referring to the Emperor of Heaven and the emperor at that time.