Saint-Étienne-du-Mont keeps the last rood screen in Paris, a carved stone bridge that crosses the nave high in the air. A rood screen split a church in two. Priests used the choir behind it. Everyone else stood in the nave, the long hall in front. Almost every French church pulled its own down after the Counter-Reformation, because church authorities wanted the congregation to see the altar. This parish never pulled its screen down. The building went up between 1492 and 1626, and the join between its late medieval east end and its Renaissance front was never tidied away.
Stand in the nave and you can see the choir straight through the screen. The stone is cut away until there is more hole than surface left. A spiral staircase winds up around a pillar at each end. Behind the screen is the shrine of Geneviève, the fifth-century woman Paris has long counted as its protector. The hill outside the door carries her name. The Revolution destroyed both the shrine and her bones in 1793. This is still a working parish, so you will often be moving round the edge of a service rather than through an empty monument.