Prevalent from the 9th through 12th centuries CE, Romanesque architecture combined the influences of Roman and Byzantine styles. The style was named, in the 1800s, because one of its key features, the barrel vault, resembled the classical Roman arch. The use of barrel vaults allowed for huge interior spaces built entirely of stone. But that also meant the roofs were extremely heavy, so the walls had to be tremendously thick to prevent buckling. Strong walls also meant fewer windows, so the insides of Romanesque churches often look dim and feel like fortresses.
Gothic architecture began in the mid-12th century with the intention of making churches look like heaven: soaring, colorful, and bright. The biggest difference from the preceeding Romanesque style was the use of flying buttresses. These support structures or towers, set off from the main walls and attached by arches, and displaced the pressure from the roof outward. Essentially, this meant the buildings could get taller, walls could get thinner, and there could be a lot of windows, which were often stained glass. Gothic churches have huge, ornate, petaled round windows called rose windows. They also are much pointier than their Romanesque predecessors, with pointed arches and tall spires (instead of blunt towers) characterizing the style.