The Pantheon in Paris (1790), and the US Capitol Building (1792-1827) in Washington DC are just two of the world-famous structures derived from Roman architecture.
Built during the reign of the Byzantine emperor Justinian (r. 527-565 CE; ), Hagia Sophia was the consummate building of its day. Outstripping even the Pantheon in size, it encompasses an enormous interior space and, like its pagan predecessor, is topped with a soaring dome. Justinian and Hagia Sophia, in fact, share much in common: both sat at the center of life in the eastern half of the Roman Empire, but both in their own way looked to the West with hungry eyes. That is, just as Justinian sought to reclaim Italy from the barbarians who had overrun it in the generation preceding (after 476 CE), the architects of Hagia Sophia strove to best the monuments of ancient Rome. Moreover, Justinian's ultimate failure to assert his rule in Italy underlay the very construction of the church itself, because in building it the emperor hoped to turn his capital at Constantinople into a "new Rome" in the East, a city which would rival and even surpass the one he tried but couldn't reclaim as his own.
But, as many an art historian will attest, neither the style nor the imagery of the Pantheon conforms with the architectural style prevalent in that day. For one, it's a masterpiece of engineering, capped with an enormous dome encasing the largest interior space the Romans ever created. Were it really Agrippa's creation, that alone would put its technical expertise decades ahead of its time. Even more remarkable, for all the violence visited on this structure across centuries of invasion and neglect, the dome has yet to fall—it's one of the few roofed buildings left intact from antiquity—so not only its design but also its strength and resiliency confirm the architect's consummate mastery of his science.
The answer to this conundrum came to light recently. Deep within the supporting structure of the dome were found bricks stamped with the insignia of a consul—a consul is a high-level Roman magistrate—who is attested to have held office many years after Augustus' reign, in the days when Hadrian, a different emperor, ruled (117-138 CE). If Hadrian, not Augustus or his retinue, were the driving force behind the Pantheon, we have an important clue in unraveling this architectural mystery.
If anything then, the Pantheon is Hadrian's architectural prayer for peace, a cultural synthesis to match the quiet and order which the Romans had imposed through force. And typical of this sage and seasoned ruler, his wish could not have been more salient or foresighted. For it was, in fact, the disenfranchised and despised, those branded "barbarian" and denied a role in Rome's triumphal pageant, who would later bring the Empire down, among them a people whose deity was never given a niche in the Pantheon, but that wasn't Hadrian's fault. These people's god insisted on having no statue.
The later Romans corrected that oversight and gave the Christian God his architectural due. The Church of Hagia Sophia in Constantinople (Istanbul in modern Turkey)—hagia sophia means "holy wisdom" in Greek—is arguably both the last great Roman structure and the first great Medieval church and, like so much of the Byzantine civilization which built it, this sacred edifice straddles time. That is, in form it looks back to the Roman Pantheon but its design is informed throughout with the sort of religious iconography which would dominate architecture in the Middle Ages to come. If no longer a Christian church—today it serves as an Islamic mosque—its structure still reveals its original purpose and reflects the age when it was first conceived.
Be it a house or a palace, library, temple or fortress, to historians there's no place like home, because few things last the way architecture does. Architectural structures have frequently survived catastrophes which have destroyed other types of evidence. For instance, when the forces of nature obliterate all human and written remains in a building, even when they eat away the façade and roof, the foundations are often preserved. Or if some party has intentionally demolished an edifice, its upper levels sometimes topple over and protect the lower ones. Moreover, because the needs of housing tend to change constantly, there's an almost continual call for new construction so that enough buildings are erected or rebuilt during every age to frame and highlight the manner in which a society has evolved.