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.
Sometimes, leather-aproned, hairs in his nose,
A unicorn is a mythological creature, a one-horned horse, an ancient symbol of virginity. 
 The comparison suggests the exotic, something mysterious and unknowable and uncontainable, just as the soul is a mystery. This flight of fancy contrasts with the literal “at one end and square”, which returns the reader to the practical task of the blacksmith.
The Forge