Celtic shield knots can be identified as any of the Celtic knots with for distinct corner areas. They usually resemble a square but sometimes they are a square shaped emblem within a circle. As in all Celtic knots there is no beginning or end. The Celtic shield knot idea comes from civilizations more ancient than the Celts. Anciently, it was a universally known symbol for protection from danger and warding off evil spirits. The symbol is always fourfold based, but within a greater unity.

"The three principal endeavors of a Bard:
One is to learn and collect sciences.
The second is to teach.
And the third is to make peace
And to put an end to all injury;
For to do contrary to these things
Is not usual or becoming to a Bard."

THE TRIADS OF BRITAIN

http://www.joellessacredgrove.com/Celtic/history.html#origins

(The Iron Age) c. 600 BC - 50 AD

Who were they? The Iron Age is the age of the "Celt" in Britain. Over the 500 or so years leading up to the first Roman invasion a Celtic culture established itself throughout the British Isles. Who were these Celts?

For a start, the concept of a "Celtic" people is a modern and somewhat romantic reinterpretation of history. The “Celts” were warring tribes who certainly wouldn’t have seen themselves as one people at the time.

The "Celts" as we traditionally regard them exist largely in the magnificence of their art and the words of the Romans who fought them. The trouble with the reports of the Romans is that they were a mix of reportage and political propaganda. It was politically expedient for the Celtic peoples to be coloured as barbarians and the Romans as a great civilizing force. And history written by the winners is always suspect.