The inner wards are based upon the power of Aeolus, the Totem bound to the Caern of the Hidden Walk. These inner wards protect the Caern in both the Umbra and the Realm, serving to keep out spirits of the Wyrm and, to a lesser extent, the Weaver. Their boundaries are roughly defined by the crevice between the north and south rim and encompass that area between them, reaching a little into the valley at both ends.
Because their power comes directly from the Totem spirit of Fog, the inner wards work by stealth, concealment, and subterfuge. Specifically, they 'deflect' attacks, redirecting or undermining the efforts of the attackers through the cunning trickery of Fog. As an example, a direct attack to the heart of the Caern might--through the power of the inner wards--somehow mysteriously 'miss' its target, the attacking enemy suddenly finding themselves, instead, directed right into an ambush by Caern Guardians. This is only a suggested example. The power of the inner wards is subtle, and not easily explained. Not everything about their function is understood, even by the Garou that benefit from them. What they do know, and have come to trust, is that they _do_ work.
These inner wards are not foolproof. They can be breached by sheer strength, or they may be pierced by cunning magic. It takes a strong bane to have much chance of piercing the inner wards. This means that the Umbra in the Caern itself is relatively safe, even by the darkest day. They are ineffective against creatures that are not spirits (meaning that Fomori and Black Spiral Dancers, for example, are _not_ affected by them). It is possible for a possessed Garou to 'carry' a bane, or other Wyrm spirit into the Caern center, effectively bypassing the wards by virtue of being possessed. However, the inner wards can and do notify the Warder, Guardians, Ritemaster, Gatekeeper, etc., of a breach by the Wyrm.