Primarily of note is that the style of shield, the hopos, was circular - representing eternity and wholeness. The shield is surrounded by the river Tethys, ocean river, the primordial realm of the subconscious from which come all images. The celestial bodies are at the heart of the shield, sun, moon, stars & constellations. The two realms of mortal men, one of peace & one of war, are at the main body of the shield; the details revealing every major aspect of human existence - unchanging since Homer's day.
From a side view, the convex shield represents the dome of the heavens over the realm of earth surrounded by the waters; the entirety of the world. Further, the two cities suggest the superimposing of the interlocked triangles; representation of the alchemical exchange that takes place throughout the epic. Most of the time a shield's exterior would be either something valued to the bearer of the shield or terrifying to the enemy; images of dragons, the gorgon, or carnage or else images of stars, wheat, or chickens were popular on shields of the time.
Consequently, it is the image of the entire world born by Achilles that is designed to protect him. The shield represents the phantasmagoria of mythology that each person bears before them to protect the bearer from the exigencies of life. It is the face to greet the faces that we meet. It also exists to disconcert the enemy; Achilles carries what he values most, human life, and puts it out at risk toward the enemy - thus disconcerting them by reminding them of what they have at risk in war.
The whole of the human experience is designed to protect Achilles who has entered the wilderness of the mind with his questions about human purpose:
"The same fate waits for the coward and the brave - both go down to the house of death, the fellow who struggles and the one who shirks"
and intensified by the loss of Patroclus and his realization that he is not "The greatest of the Achaeans" but rather a "worthless dead weight on the good green earth." This is similar to what Plato describes in Phaedo when examining the reincarnation of souls:
τίς μηχανὴ μὴ οὐχὶ πάντα καταναλωθῆναι εἰς τὸ τεθνάναι;
"is there any escape from the final result that all things would be swallowed up in death?”
- Phaedo 72d
The same examination that appears in Ecclesiastes
Vanity of vanities, saith the Preacher, vanity of vanities; all is vanity.
What profit hath a man of all his labour which he taketh under the sun?
One generation passeth away, and another generation cometh: but the earth abideth for ever.
The sun also ariseth, and the sun goeth down, and hasteth to his place where he arose.
The wind goeth toward the south, and turneth about unto the north; it whirleth about continually, and the wind returneth again according to his circuits.
All the rivers run into the sea; yet the sea is not full; unto the place from whence the rivers come, thither they return again.
All things are full of labour; man cannot utter it: the eye is not satisfied with seeing, nor the ear filled with hearing.
The thing that hath been, it is that which shall be; and that which is done is that which shall be done: and there is no new thing under the sun.
Is there any thing whereof it may be said, See, this is new? it hath been already of old time, which was before us.
There is no remembrance of former things; neither shall there be any remembrance of things that are to come with those that shall come after.
This struggle is epidemic to the human race - terrifying and yet necessary if we are to mature. If we are to grow up, become autonomous, we have to face this "dragon" of our own worthlessness. The shield, the images and stories we hold most dear, help serve as a protection against those monsters that threaten to consume us, drown us, devour us, and overwhelm us.
Shield of Achilles (Ends of the Iliad and Beginnings of the Polis) by Nagy
The Shield of Achilles and the Negative Future Perfect by Chiasson