What major architectural project did King Louis IX of France build?

King Louis IX of France, also known as Saint Louis, is famously known for commissioning the Sainte-Chapelle, which is one of the finest examples of Gothic architecture. Built between 1242 and 1248, the chapel was designed to house royal relics, particularly a collection of Passion relics, including what was believed to be the Crown of Thorns. The architectural design showcases stunning stained glass windows, which beautifully depict biblical scenes and allow for an ethereal light to fill the space. Sainte-Chapelle not only served a functional purpose in housing sacred artifacts but also symbolized the power and piety of Louis IX, reinforcing his devotion to Christianity and his status as a revered monarch.

Other options do not pertain to projects associated with King Louis IX. The Eiffel Tower, constructed in the 19th century, was built long after his reign. The Palace of Versailles, a significant royal residence, was commissioned by Louis XIV, not Louis IX, and Notre-Dame Cathedral, while famous and integral to Parisian architecture, was largely completed before his reign, although he did contribute to its maintenance and reverence.