The Celtic Tree symbolised the balance and harmony within all forces of nature through the interaction of different energies of the Universe. Trees were worshipped for their wisdom, strength and longevity.
As the Celtic Tree of Life reaches up towards the stars, the Sun and the Moon and into the universe and delves down below the lower world before meeting up again, like Celtic knots, it has no beginning and no end. The Celts believed the crann bethadh to have special powers. When clearing a piece of land for human settlement, the Celts of Ireland would leave a tree at the centre. They would hold assemblies and festivals beneath this tree.
Druids could traverse the three Worlds by climbing the Tree of Life into the Upper World of stars and celestial bodies (the dwelling place of many Gods, the great Mother Goddess and spirits of the air.) They would descend through its roots into the Otherworld ruled by the Lord of the Underworld or the Horned One and Protector of Animals. This is where Druids would meet with their power animals and spirit guides.