Windows as openings in the walls in order to let light in were first discovered with the egyptians. They mostly used animal hide, paper or fabric to cover those holes in the walls to protect themselves from wind and other natural forces, but were still able to enlighten the room. But later in the antique empire (around 100 AC), the romans managed to produce glass and build real windows. The glass was blind at first because it was only smooth on one side and rough on the other. Around 200 AC, they discovered a new technique where the glass was smooth on both sides so their windows weren’t only there to let light in anymore, but also to be able to look outside. The romans mostly built round windows, because they were only able to produce round glass plates instead of rectangular ones. In Old High German, the term for window was ‘Windauge’ (wind + eye) which was an opening in the upper parts of the walls of the building to let out smoke from the wooden stove inside and provide some air exchange.
In the Romanesque era (ca 1000-1250 AC) glass windows were found scarcely and natural light was not considered important inside of buildings. Only later in the 12th century, when the gothic style of building was discovered, windows started to be more popular. The thought was to build in a way so people could be closer to god. So the churches were built higher to get closer to heaven. The Romanesque style was not able to do that. The walls were too thick and heavy and the round arches, that were typical for that time, could not carry the weight of such high towers that were intended to be built. This was the cause for some new churches collapsing before they were even finished.
In the gothic style though, the arches were pointed and slimmer than the prior ones. They could carry more weight and both the walls and arches could be built lighter than before, which made it easier to build high towers. Apart from that, light was also considered to be essential, because the more light came into the church, the closer it was to god. So many windows were put into the walls. They were made out of stained glass and painted so they would display biblical stories. The windows were built out of smaller pieces of stained glass that were connected with lead. This also brought the advantage of the glass not breaking in case of a slight earthquake. The light that fell into the room was full of different colours which also supported the supernatural aura of the building. Glass still remained a rare and expensive privilege that was only available for the wealthier part of the population.
In the beginning, the windows in smaller houses were only non-glazed sliding windows. Later they were glazed but soon to be replaced by casement windows like we know them today. For a long time it was usual to have windows with glazing bars, because rectangular shapes glass was only available in small sizes. With that it was possible to have large windows without having to produce single glass plates in that size. Only in the 60s when the ‘Float-Glass’ was introduced, it was possible to produce those. This procedure was revolutionary for the glass industry and replaced most of the other ways of producing glass plates as well as the windows with glazing bars.