The Latin cross plan takes as its model the Roman Basilica. It consists of a nave and two sections half its size flanking it, and the altar stands at the far end (the short end or "head" of the cross). Also, cathedrals influenced or commissioned by Justinian employed the Byzantine style of domes and a Greek cross (resembling a plus sign), centering attention on the altar at the center of the church.
Main article: Romanesque architecture
Before about the 12th century, cathedral-builders used the Romanesque style. This style put a lot of stress on the walls, forcing them to become very thick and without windows. The name given to the era of these churches, the Dark Ages, seems apt. Rounded arches, a Roman invention in architecture, provided openings. Romanesque buildings had very little adornment.
Main article: Gothic architecture
In the 12th century Abbot Suger introduced the flying buttress, which proved a great innovation in supporting buildings. Beams came out and down from the building, resting much of the weight on the ground outside. The walls could then become thinner and even have windows. The windows installed contained beautiful stained glass, showing stories from the Bible and from lives of saints. The pointed arch provides another trademark of the Gothic style. Such new elements of design allowed cathedrals to rise taller than ever, and it became something of an inter-regional contest to built a church as high as possible.
Surviving examples of medieval secular architecture mainly served for defense. Castles and fortified walls provide the most notable remaining non-religious examples of mediaeval architecture. Windows gained a cross-shape for more than decorative purposes: they provided a perfect fit for a crossbowman to safely shoot at invaders from inside. Crenelated walls (battlements) provided shelters for archers on the roofs to hide behind when not shooting.
Elements of Medieval Architecture