King Louis IX paid 135,000 livres tournois for the Crown of Thorns in 1237, a sum that exceeded what he would spend building the chapel to house it. That chapel, Sainte-Chapelle, was designed less as a church than as a giant reliquary: 42.5 metres high but only 36 metres long, its proportions deliberately vertical to draw the eye upward toward the sacred treasure at its heart.

The Lower Chapel served palace staff and commoners, its blue ceiling gilded with fleur-de-lys. Most visitors rush past the Virgin and Child statue at the entrance doors, yet it remains one of the chapel's finest sculptures. The Upper Chapel, reserved for the king and his court, is where 1,113 stained-glass panels flood the interior with coloured light. The relics themselves moved to Notre-Dame during the Revolution and famously survived the 2019 fire.

VoiceMap's self-guided audio tours trace the Crown of Thorns from Constantinople to Paris, explaining how medieval kings used sacred relics to cement political power and why proximity to them promised eternal salvation.