A Late Antique ivory carving
Roman art, German art, and Islamic art mixed together in the Early Middle Ages to create a new form, which we call medieval art. Medieval art is usually divided into several different kinds, each of which were expressed differently in different countries, and which even happened at different times in different places as well.
First is Late Antique art, where some people made art in a more or less Roman style, so people would know they were Roman Christians, while other people were making consciously German art, so people would know they were Germans and Arians. In Spain and Constantinople, Late Antique art lasted until the Arab conquest of Spain in 700 and the iconoclast controversy in the 700s AD. In France, it lasted longer, until the end of the Merovingians around 800 AD. In England and Germany, still later, until about 1000 AD. In Italy, where people were proud of their Roman heritage, they kept making Late Antique art until about 900 AD.
Moissac, 1100s France
After Late Antique art came Romanesque art, where German, Roman, and Islamic elements mixed and brought a new energy and excitement to architecture, sculpture and painting. Romanesque architecture has a heavy, substantial, impressive feel. Sculpture is very abstract, combining stiff, formal forms in some places with fantastic carvings of real and imaginary animals and demons in others. Nearly all Romanesque art was of Christian themes.
Romanesque art in northern Spain, where Romans and Visigoths fled after the Arab conquest, was very much influenced by Islamic art. In France, Charlemagne got a lot of good Romanesque art produced around 800, and then Charlemagne's children and grandchildren also paid for a lot of art. In England, Romanesque art came from France with the Norman Conquest in 1066 AD. In Germany, the Holy Roman Emperor Otto the Great, also about 1000 AD, got artists to being working in the new Romanesque style.
Romanesque art gradually turned into Gothic art around 1100 AD in Italy, and then spread slowly over Europe between 1100 and about 1300 AD. The easiest way to recognize Gothic architecture is by the pointed arches, but in general Gothic architecture shows a new ambition to reach higher and lighter, with walls of glass, reaching up to Heaven. Gothic sculpture shows more concern for realism than Romanesque, and more emotion. Now Mary appears everywhere, and Jesus becomes more merciful, compassionate, loving. In painting, artists also searched for more realism and more emotion. Artists like Giotto and Fra Angelico began to experiment with backgrounds and crowd scenes.
In Constantinople, Gothic never caught on, because people thought it was a Western, barbaric style, and because iconoclasm made it seem wrong to paint people too realistically. In France, by 1200 people began to build Gothic cathedrals, and in England (now heavily influenced by France) not much later. In Germany and northern Spain, Gothic came later, not until around 1300, and in southern Spain Islamic styles came with Islamic government until almost 1500 AD.