Collégiale Sainte-Gertrude.

Standing as a definitive example of Ottonian and Romanesque architecture, this massive collegiate church is defined by its unusual double-choir layout and a central nave stretching over 100 meters. While its origins date back to a 7th-century abbey founded by Itta of Metz, the current structure is a feat of 11th-century engineering, reconstructed with care after sustaining heavy damage during WWII. Inside, the starkness of the Romanesque stone is balanced by intricate 18th-century wood paneling and marble sculptures by Laurent Delvaux. Below the floorboards, the archaeological basement reveals the foundations of four predecessor churches and the largest crypt in Belgium. For those willing to climb 132 steps, the "Imperial Room" in the westwork offers a closer look at the building’s fortress-like geometry and the octagonal belfry that dominates the Nivelles skyline.