Windows of the Romanesque Middle Ages typically have a semi-circular arch in the brickwork, which constructively holds the masonry in place. The semi-circular arch is known as one of the most simple and stable constructions throughout history.
The Romanesque window is typically a narrow window with lead lights set in a wooden or iron frame. Lead lights are fragile, so iron rods are attached across the window at various points, to stabilize the construction.
The Romanesque churches are very often built with solid masonry, and the window reveals are therefore slanted to "pull" more daylight into the building.