1) His purpose plain = Mankind's real purpose in life is made plain through music-- presumably a noble purpose: the creation of Beauty.
2) empty faces pale-- the spiteful, the stingy and the rude: that's at least three faces, hence the plural.
3) my towers = the things (physical and intellectual) that I have built during my lifetime.
4) Spell-bound= (this is an abstruse one, NH) perhaps: let me (the city) live entranced by music (until Doomsday).
- For Teachers