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Sainte Chapelle is a must see on any trip to Paris. It was built in 1239 by Louis IX to house ancient Christian relics, including the crown of thorns (which was subsequently moved to Notre Dame Cathedral after its completion in 1345).
Sainte Chapelle in Paris is one of those rare places that really does look as beautiful as it does in the postcard perfect photos that are posted after peoples visits. The closest comparison that I can make to the feeling of being inside Sainte Chapelle is to being inside a rainbow.