Just a few miles from Siena, the Tuscan countryside’s contours of farm and vineyard are broken by a perfectly shaped conical mound, crowned with a grove of cypress trees. A stone pyramid on its summit, visible from every approach, bears simple white plaques on two of its sides. One reads ‘MCCLX’, for 1260, and on the other, ‘Montaperti’, or ‘the hill of death’. Montaperti is the site of a major battle between Florence and Siena, the bloodiest day in the long medieval wars among the Italian city-states. The Florentines were routed, with 10,000 of their men left dead on the field. Casualties numbered mere hundreds on the Sienese side.

The construction of a memorial fit for a Pharaoh dates from 1921, an echo of Europe’s, and Italy’s, 18th- and 19th-century love affair with Ancient Egypt, that heady mix of esoteric Freemasonry and an elite imagination fired by Napoleon’s campaigns in Egypt from 1798 to 1801. Egyptian artefacts flooded back across the Mediterranean in his wake, and more were to follow; the collections of Egyptian artefacts in Europe’s great ‘world’ museums alone amount to hundreds of thousands of objects, comparable to the legacy of Greco-Roman Antiquity. Modern Tuscany is peppered with sites similar to Montaperti: the half-Florentine artist (and incidentally, a mason) Frederick Stibbert turned his garden and villa in the city into a pharaonic suite with a gloomy temple, set beside a river (to stand for the Nile), and surrounded by obelisks and sphinxes.  And the aesthetic appeal of Ancient Egyptian forms, suggesting primeval grandeur and power as well as solidity, the prospect of an eternal home, endures. Silvio Berlusconi, almost two decades before he died, and then only into his second term as Italy’s prime minister, commissioned a tomb from the celebrated architect Pietro Cascella: a crypt smothered beneath an enormous pile of marble adorned with pyramids.

Siena lies to the west. On bright days, Siena’s highest towers, the cathedral and the Torre del Mangia above the Palazzo Pubblico, can be seen clearly, winking in the light of the sun, hovering, as if by illusion, just above the horizon.

The significance of Siena’s victory over Florence is two-fold: it is a living symbol of Sienese pride, a definitive point in history at which Siena triumphed over its larger and more celebrated neighbour, an event of such intensity that it was thought worth the disturbance of the Florentine dead the better part of a thousand years later. Contemporary sources testify to the victory as a special example of the Sienese people’s devotion to the Virgin Mary, and of her successful invocation in defence of the city.

During those long decades when trade flourished across the Italian peninsula, secular and religious splendour flourished in parallel. Looming over Siena’s main square, the magnificent Torre del Mangia testifies to Siena’s growing opulence and ambition. It was completed just a year before the onset of the Black Death in 1348. The still unfinished nave of Siena’s cathedral, just a few hundred yards away, bears eloquent testimony to just how irrevocably the Plague transformed Sienese society. On visiting the city in 1590, the French philosopher Michel de Montaigne remarked that Siena was ‘uneven’, its streets spilling steeply down valleys away from the ridge on which its civic centre sits. He also remarked upon ‘its appearance of great antiquity’. Siena had changed very little over the previous two centuries, and indeed, little has changed since, and the splendour of the whole city’s medieval architecture remains unsurpassed. Siena had long benefited from its position on pilgrimage routes from northern Europe to Rome, such as the via Francigena. And as the rapidity and reliability of trade grew, precious metals and gold leaf were traded enthusiastically, and luxurious textiles from the East appeared on the markets that lined the old Roman roads that branch through the city. Events far off in Asia and on Europe’s fringes gave trade an element of predictability: the Mongol Conquest had brought some stability to the Silk Roads, and merchants could travel freely under the new dispensation.

Siena’s civic and religious functions, the inherent dynamism of its economic life, as well as relative political stability under its innovative republican administration (election by lot from a small pool of influential citizenry who had a large stake in maintaining the good health of the city) is amply demonstrated in the murals that adorn the Palazzo Pubblico at the heart of the city. In its main hall, Simone Martini’s Maestà (1315) has the Virgin with the apostles nestled under a vast and luxurious canopy. The fresco is ringed with the mottos and arms of Siena – a common home protected by its Marian devotion. In a chamber opposite, Ambrogio Lorenzetti’s fresco Good and Bad Government (1338-9) contrasts the ideal city, where maidens dance hand in hand in the streets and merchants ride freely through the surrounding countryside, with the hell and anarchy introduced by poor leadership – fire, rapine and disease stalk the land while the devil looks on, pleased with his work. The visual arrangement of the former situates Siena’s profane grandeur, the fine drapery covering the crowd of saints, in dialogue with an eternal community of the spirit; the latter points to the fragility of the civilised world, coded in the miniature separations in space between an age of tranquil advancement and a time of monsters.

These decades, truly a ‘golden age’ of late medieval Sienese society, are given a thorough and at times spectacular treatment in London’s current National Gallery exhibition, Siena: The Rise of Painting 1300-1350, the fruit of a decade-long collaboration between New York’s Met and the Paris Louvre. Its achievement is to capture key developments, in elegantly curated snapshots, of the early textures of Western Europe’s ascendancy, its increasingly sophisticated commercial culture, the foundational influence of powerful and independent-minded city-states, and the first stages in the development of the expansive range of media, styles and symbolic building blocks which make up the idea of ‘painting’.

Works by the four greatest painters of the Sienese trecento – Simone Martini, Ambroglio Lorenzetti and his brother Pietro, and Duccio – are threaded through the exhibition. It is Duccio who sits at the physical and symbolic centre of the exhibition. Both the murals by Martini and Lorenzetti in Siena’s Palazzo Pubblico are difficult to imagine without Duccio’s influence – and both are believed to have been employed in his workshop. His Maestà, a double-sided altarpiece constructed for Siena’s Duomo between 1308 and 1311, appears here for the first time in as complete a form as it has been in for centuries. Split into dozens of fragments and distributed across the world’s galleries, here the Maestà, with both its back and front predella, is virtually reconstituted as it might have appeared to the devout of Duccio’s time. The altarpiece’s many panel paintings represent an ambitious expansion of sacred space, as if another chapel had been constructed in and around the altar.

The various rooms – in which the Last Supper and Christ’s trial and condemnation take place – each have dark doorways, artfully positioned to expand the depth of each scene. One of the instantaneous emotional responses the panels provoke is claustrophobic intensity, the horrifying pace of Christ’s betrayal and execution. And the formal relationships between the panels suggest a personal vision at work – for Christ is a God who feels, like us, the blank horror of the mob, the ominous mutterings and shuffling of feet from a farther room. Duccio triumphs, as the art historian Otto Pächt observes, by ‘smuggling the time factor into a medium which by definition lacks the dimension of time’. The Maestà represents nothing less than an expansion of the world perceptible to the artist, especially in comparison to the Byzantine models that had for centuries set the decorative and moral standard. The typical viewer of Duccio’s own period would have felt the urgency of his vision, the relevance of the moral message to the daily life of his city. In the panel depicting Christ’s entry into Jerusalem, the grandeur of Herod’s city, redolent of worldly excellence and wealth, inevitably leads the viewer to reflect on Siena’s solidly built many-sided towers. The Maestà rewards attention, even provokes the religious mind. On the footrest of the Madonna, Duccio signs his work, attributing the panel paintings to his own workshop, a novel and unusual gesture. The radicalism is striking – for the suggestion is that what one set of eyes sees in one way, there may be a thousand eyes, a thousand ways of looking.

This essential step – the expansion of temporal and spatial possibility – is powerfully demonstrated throughout the rest of the exhibition. The effect on the applied arts is fascinating, notably in the chalices commissioned by great patrons in Siena and throughout the Italian peninsula. Sienese goldsmiths worked for clients throughout Europe: one of their efforts, commissioned by the Papal court at Avignon – an Agnus Dei wax container of stunning compression and delicacy – has a pleasing stylistic consistency with the more complex commissions positioned alongside it, such as reliquaries incorporating many minute, panelled scenes. Certain floral patterns which appear and re-appear among the displays reveal a tender interest in the natural world and why they please the eye, a questing ethic that asks of us: how can we decode the language of silent things? What is the inner significance of Nature’s abundant work and arrangements?

Most revealing is the way in which taste shifted during the period. Many of the early panels bear features designed to provoke correspondences between the pigments and precious metalwork and gold leaf. Because of its relative scarcity, metalwork bore a higher status. Duccio’s Maestà was itself commissioned as a money-saving device – far cheaper than an altarpiece loaded with precious metals. Look carefully for the sgrafitto technique in the panels where the gold leaf is scratched to give the impression of chunkiness and solidity. What fascinates about the transformation wrought by Duccio and his successors is the weakening relationship between these physical effects and the acquisition of symbolic power. The Maestà delights not because it feels and looks like a precious object; instead, the work’s tradeable value increases with the use of line, storytelling, and the dextrous play of ideas. Duccio suggests a promise of inner complexity, of a broader range of psychological responses that carry beyond the moment. The marvellous and playful works of the next generation build gorgeously upon this style. They amount to a compelling argument for the deep integration of Siena’s cultural confidence and its civic and economic vitality, and its special significance in the history of art. By contrast, although deeply sophisticated and a thriving economic force, the contemporaneous Novgorod Republic in the far north had its own visual and decorative arts, but its artists never moved beyond the Byzantine models in the two centuries that preceded the state’s definitive absorption into Moscow’s lands – a dramatic gulf in sensibility that arguably still underlies the relationship between Russia and the West.

One of the moving features of Siena’s trecento art, to 21st-century eyes at least, is its elision of utility (these panels were after all the objects of daily veneration and ritual practice) and aesthetic charm and sophistication. Never such innocence again – for to speak of the ‘rise of painting’ suggests that there has been, or might be, a ‘fall’. Try adopting the perspective of a 21st-century tourist who looks back towards Siena from the pyramid on the hill at Montaperti. So many centuries of rampant experiment and dynamism in the western arts have led to its existence – how else is it possible to explain the choice to echo an Egyptian spiritual tradition that extends backwards in time for thousands of years, transmuted and transformed into a 20th-century burial mound for those who had died centuries ago under the banner of an Italian city-state? The impression left is one of grandiloquence, suggestive of the exhaustion of human perception rather than its expansion. Siena’s art brings the 21st century citizen into contact with an age much more materially limited than our own and yet still capable, so many centuries on, of kindling a bright flame of inspiration, power, and self-knowledge in any mind receptive to its message.