The Art and Architecture of Medieval times encompasses many movements, or eras, in art history. Included in this period are the Early Christian, the Byzantine, the Carolingian, the Romanesque, and the Gothic periods of art and architecture. The Medieval period is complex, but a good overall review of the distinctive natures of each era can be found at Age of Discovery – Medieval Art – Renaissance – Exploration. An excellent overview of this period is at Early Medieval Art, an extensive art history site that covers all areas of art history.
A good start in the study of Medieval Art and Architecture is to understand the vocabulary. The University of Pittsburgh has a glossary of Medieval Art and Architecture at Glossary of Medieval Art and Architecture. Another source for basic information is Medieval Art and Architecture: Western Europe 400-1500.
The most significant artworks of the Early Medieval period are the illustrated manuscripts, books that were written by hand on handmade parchment paper made from animal skins. The monks who spent their lives producing these priceless treasures painted the beautiful illustrations and initial letters. For further information, as well as photographs of these books, see The Art of the Book in the Middle Ages, produced by the Metropolitan Museum of Art. Perhaps the most famous of the illustrated manuscripts is The Book of Kells. Photos and information can be found at Book of Kells.
The Byzantine art and architecture can be seen at various websites. Byzantine Art has photos of some of the most famous of medieval architecture, mosaics, and sculptures. The Virgin Mary was represented frequently during this period, and many Byzantine art pieces can be viewed at The Cult of Mary in the Middle Ages.
The next era of Medieval Art and Architecture was the Carolingian period. Early Medieval: Merovingian, Carolingian, Ottonian displays a few examples of this work, and Images from World History: Carolingian synthesis: miniatures has some excellent photos of the miniatures popular during this time.
The Romanesque era of the 11th and 12th centuries is well known for its architectural elements, examples of which can be studied at Romanesque Art. Churches and cathedrals became highly decorated with painted frescoes of Biblical scenes and elaborate architectural elements. Metalsmithing was at its peak, and its craftsmen made many decorative scenes and utensils for these churches. Another source of information is the Corpus of Romanesque Sculpture in Britain and Ireland, an organization with information on the Romanesque period.
Many of the French cathedrals can be seen at the University of Pittsburgh’s site, Medieval Architecture in France. One page of that site, Medieval Saint-Denis Home Page, shows many detail of the St. Denis Cathedral. Medieval Paintings in the South of France also is a good source for information on many aspects of the French Medieval time.
In Britain, examples of the architecture and stained glass windows of the Gothic period can be found at English Gothic. French Gothic examples are at Medieval Art & Architecture: French Gothic.
Other sites show various castles built during the medieval period. One source, Medieval History, Castles, lists several WebPages with sources.