The anvil must be somewhere in the centre,
The fact that the anvil has to be in the “centre” shows the importance that the blacksmith puts on the anvil. And since he later refers to it as an “altar” relates to the fact that in religious contexts the item of importance, the altar, is place in the center so that it can be the center of everything in order to honor the item of worship.
Since it is also described with the word “unicorn” gives the anvil a fantastical and mystical quality which also shows that the blacksmith puts much weight behind the anvil and its importance.
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