Yes, I might have exaggerated with bonuses. Then, let us forget about armour.
The point remains quite the same. It should explore the whole preservation theme and should even last very long, at the same time. In short:
preservation and
duration (Rivendell was kept safe for hundreds of years).
If the Ring influences units, heroes and buildings across the whole map, it would work quite similarly to Nenya (global range) and its effect could not last very long, due to balance. One of the revolutionary aspects of my concept is that the focus is only on one precise place, referencing the status of Imladris as a secluded, merry valley. Furthermore, any negative side-effect on enemy troops should also be avoided, because none of the Three is meant to harm or interfere (directly) with your opponent.
The idea of nullifying enemy spells and heroic abilities I consider the starting point. It is what we have connected to the act of preserving something. Such a smart implementation, I would say. Yet, I wonder if this suffices for an ultimate ability. Surely, the advantage of having a single target available (the citadel) allows for a definitely longer duration.
I would then rephrase it this way, as a sort of skeleton-concept. We can add other effects or raise values in a later moment
Vilya: Vilya shields Rivendell from decay and evil. All buildings, units and heroes within the castle are immune to enemy spells and heroic abilities for a very long duration.
It means that destructive spells or weather-altering phenomena are going to be rendered useless, for a quite long time, if cast on your fortress. It signifies that, behind your walls, you won't incur grave danger.
I know that it may sound constraining, but global effects cannot last adequately and I'm even making an attempt to differentiate Vilya from Nenya. To distinguish them as it's due.
EDIT: Graphical renditions can stay the same. That is, the aforementioned 'saint-city'.