The Solar cross is probably the oldest religious symbol in the world, appearing in Asian, American, European, and Indian religious art from the dawn of history. Composed of a equal armed cross within a circle, it represents the solar calendar- the movements of the sun. Sometimes the equinoxes are marked as well, giving an eight armed wheel. The swastika is also a form of Solar cross. The sun cross in its most simplified form is known in Northern Europe as Odin’s cross.
The Celtic shield is often used to ward off evil spirits, and also for protection. Many people across the world have received tattoos or talismans of this object so that they can do this. Usually in this symbol it is always square, possibly to represent the 4 elements, earth, air, fire, and water. Sometimes in Celtic artwork you will see this symbol drawn in more detail with many more knots or more complex. This is easily seen throughout ancient Mesopotamia.