Rho Aias

Seven Rings of Heaven.
In Greek mythology, the shield used by the hero Aias during the Trojan War.
It was a bronze shield covered by seven layers of ox hide, and the only thing that proved capable of stopping the javelin of the great hero of Troy, Hector. (Though it managed to penetrate six layers of ox hide, the seventh layer was unbroken.)
Afterward, its fame spread, and its existence was eventually sublimed into a “conceptual armament” boasting absolute defensive power against thrown weapons.
It is the only defensive armament Archer can use proficiently.
The Rho Aias that protected Shirou’s spellcasting during the finale of the Rin route was not something he projected himself, but was actually made by Archer who was watching the battle from a distance.

Fate/side material: Fate Encyclopedia