All I know is a door into the dark.
Outside, old axles and iron hoops rusting;
Inside, the hammered anvil's short-pitched ring,
The unpredictable fantail of sparks
Or hiss when a new shoe toughens in water.
The anvil must be somewhere in the centre,
Horned as a unicorn, at one end and square,
Set there immoveable: an altar
Where he expends himself in shape and music.

 The fact that the anvil “must be somewhere in the centre” reinforces its alchemical symbolism. The anvil is the central force from which the  horseshoe/soul emerges from the “door into the dark”; an echoe of the first line. 
 So the work done by the blacksmith is to shape the metal and eliminate its impurities and rust. The anvil is a tool of refining not only the metal but also the human self, represented by both the blacksmith and the poet.
The Forge