What then is the wheel...and how does it relate to Ada-Mantia etc.?
Many thanks in advance...
It's a metaphorical description of the shape of the universe. Sort of like http://upload.wikimedia.org/wikipedia/commons/thumb/b/b9/Atom_of_Atheism-Zanaq.svg/709px-Atom_of_Atheism-Zanaq.svg.png incorrect, but representative picture of an atom.
Nirn is the hub of the wheel, and the Aedra sacrificed themselves to become its supporting spokes. The outer rim in Aetherius and the negative space inbetween is Oblivion.
Ada-mantia is part of a network of other Towers that mimics this pattern, and mimicry has very real power.