As the Roman Empire distintegrated amid war and rebellion, living standards in western Europe declined precipitously, and so did the population - to a third of what it had been. Poverty and lawlessness were universal. Marauding pirates converged on the West from Africa, Scandinavia and the Russian Steppes, pillaging deep into the European heartland. The Eastern Roman Empire retained its central organization, and even expanded for a while under Justinian before being decimated by the Moslem armies. Though Christianity survived in what is now Greece and Turkey, centuries were spent simply battling for survival. For obvious reasons, the castle was the dominant architectural form in these times. Perched on crags and surrounded by defensive moats and battlements, the castle provided a base from which its occupants either plundered the countryside or heat off attacks from rivals.

Stone churches were rare in western Europe, and towards the East most builders reused the Roman materials close to hand, such as bricks and broken marble. The few churches that remain from this period are beautiful in their simplicity. Lacking in pomp and fashionable gloss, they are among the purest examples of architecture left to us in the history of architecture. A good example is Germigny-des-Pres in central France, which was designed as a cube surrounded by four semi-cylindrical chapels with domical vaults. These perfect Platonic solids are juxtaposed in a lovely but unostentatious arrangement. It is an architecture of purity and meditation within the surrounding storm.

In Constantinople in the 4th century AD, Justinian's architects built the largest church ever known, Hagia Sophia, or the Church of Holy Wisdom, as massive and awe-inspiring as the pomp of his expanded Empire. The charming little church of SS. Sergius and Bacchus nearby was a model for the greater building.

When Charlemagne followed Justinian's example centuries later, and recreated the Roman Empire in the West, he naturally looked to the East for inspiration. His palace church at Aachen (AD 796-814) is also a centralized building, symmetrical in every direction but proportionally much taller. The soaring height foreshadows later developments in Western architecture.

The basis of this architecture was the wall. As in all earlier periods, it was the thickness and weight of the walls that kept the vaults, the massive domes and the roofs in their place; openings were simply holes cut through the walls. The bigger or the taller the building, the more massive the walls and the smaller the openings. It was a stable architecture - weighty, permanent and secure - in direct contrast to the uncertain times in which it was created.