Palladium

In Greek mythology, the Palladium was a wooden statue of the goddess Athena (or Pallas) that protected Troy.

Its theft by the Greeks enabled the city's fall.

By extension, a palladium is anything believed to provide protection or safety for a community: Free speech is the palladium of our liberties.

This sense appears mainly in elevated or literary contexts. (Lowercase palladium is the chemical element.)