Rho Aias

The Seven Rings that Cover the Fiery Sky (熾天覆う七つの円環) is the shield used by Aias (Telamonian Ajax) in the great war of the Greek myth, the Trojan War.

This bronze shield covered with seven layers of oxhide is the one that blocked the javelin of the great hero Hector that was never blocked before. It penetrated 6 layers, but the seventh resisted.

Afterward, its existence was sublimed into a Conceptual Weapon that was an absolute defense against any thrown weapon. It is Archer's strongest defense.

At the end of Tohsaka Rin's route, the one that defends Emiya Shirou isn't projected by him, but by Archer from a distance.

Source: Fate/Side Material encyclopedia entry (p.78)