Yggdrasil's structure, as the Prose Edda describes it, is genuinely cosmological rather than decorative: an ash tree whose branches and roots physically connect the nine worlds of Norse cosmology, one trunk holding the entire structure of the universe together rather than standing beside it as a separate sacred object. The Prose Edda names specific beings tending it, the Norns, who draw water from the Well of Urd at its base to keep it alive, and specific beings gnawing at it, most notably the serpent or dragon Nidhogg at its roots, so the tree in the source text is not a static emblem but the site of ongoing maintenance and ongoing damage, a structure actively held together against forces working to unmake it.
The Well of Urd beneath the tree is also, in the Prose Edda's account, where the Norns cast lots and set the fates of gods and men, tying the tree's physical maintenance directly to the working of fate itself rather than treating the well as a separate, unrelated feature of the cosmological landscape. Among the nine worlds the tree's branches and roots connect, the Prose Edda names Asgard, home of the Aesir gods, and Midgard, the world of humans, alongside realms associated with giants, elves and the dead, so the tree's structural role is not confined to linking two or three principal realms but to holding together the entire nine-part cosmos as a single continuous structure.