IMG and IMStory, Than Ulbar has a powerful (six mastery or more)
guardian.
The Wall outside is part of it: it is made of millions of skulls,
about half filled with mad head ghosts.
Defense and Awareness issues are pretty much covered by the five
mastery barrier of thanatari skulls-ghosts.
Other defenses are managed by demons and magics and barriers inside
the walls.
The blessing part of it is that which sustains mr. Xsa Glut himsef.
I mean, you need not to abstract the power of the 'city go' as a
single manifestation of magical power. You could (I did) break it up
in several different phenomena, which the heroes have to overcome
one after the other. And those powers, unless the city is without a
leadership, are directed by the Ruler or Rulers of the city. So they
are changeable, up to a certain grade.
I doubt ancient and powerful cities, like Old Pavis against Jaldon
or Furthest against Argrath or Than Ulbar against the Clessidrans,
have three static powers with high numbers. There must be a grade of
flexibility, managed by the inside City God Cult or Government.
Else any Gloranthan City would fall after the first or second failed
(and improvised) siege, because its Guardian is static and the
Conquerors are not, by definition.
Gian