And I discovered that my castles stand
Upon pillars of salt and pillars of sand

from Coldplay – Viva la Vida Lyrics on Rap Genius

Meaning

Biblical reference. A man who builds his castle on sand or salt will fail as they are a weak foundation. By contrast, the castle built on a on rock (viz. God) is durable

The speaker recognizes that his faith was in his own ego rather than something greater.

Also, salt refers to Lot’s wife who turned into a pillar of salt when she looked back on Sodom as they fled the city

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