Begun in 1228, just two years after the saint’s death, the basilica of San Francesco represents one of the most astonishing feats of medieval architecture, in terms both aesthetic and technical. The name of the architect is unknown, though it may well have been the combative, domineering Fra Elia di Buonbarone (1171-1253), Francis’ successor as Vicar general of the order, who actually inspired the design. It was certainly he who set the work on foot, armed with the blessing of Pope Gregory IX who had canonized the saint. The operation of building two great churches, one above the other, was made far more difficult by the site, a rocky precipice on the fringe of the city. Undaunted, the builders created a system of massive arcades underpinning the whole structure, as its most impressive when viewed from a distance and completed the entire basilica with the aid of donations pouring in from as far away as Jerusalem and Morocco, in a mere thirty years.

Within this unique creation, powerfully marked as it is by the influence of French gothic churches (so much so that a French provenance has been suggested for the anonymous architect), the finest painters of the age were commissioned to decorate the walls and vaults with lavish frescoed panels which have since come to represent a kind of basic grammar in the language of Western art. Much as the early friars may have represented Elia’s project for what seemed more like a nobleman’s palace than a memorial to their Christ-like founding father, they can hardly have objected to so splendid a celebration of the Franciscan spirit in the wider context of Christianity.

In the Lower Church, which you enter through Francesco di Pietrasanta’s Porch of 1487, sheltering twin gothic doorways with a rose window above them, you will find frescoes by five major Tuscan artists at work during the early fourteenth century. Immediately to your left are perhaps the most beautiful of all, those by Simone Martini, appropriately adorning the chapel of San Martino and representing scenes from the saint’s life. Painted in 1317, they show the influence of Giotto in their realism and handling of brilliant colours, while retaining the poetic delicacy so typical of Simone’s native Sienese style. Contrast, for example, the chivalric splendours of St. Martin Renouncing his Life as a Soldier all tents, lances, warriors and steeds, with the dramatic variation of the figures, some praying, some tenderly solicitous, posed around the saint’s deathbed.

In the left transept another Sienese, Pietro Lorenzetti was engaged in 1320 to portray scenes from Christ’s Passion. Here it is not just the tonal subtleties of the artist’s palette which catch the eye, or the sense of fluid movement in the composition, but his interpretation of every moment in the story in terms of complex, mingled reactions from those present. In the Entry into Jerusalem, how confident the disciples look, how eagerly the children scramble to throw down their clothes, yet what a quietly sinister note is struck by the two bearded Pharisees standing conspiratorially in the crowd.

The vaults of the sanctuary were always said to have been frescoed by Giotto, but scholars now believe them to be by a pupil known therefore as ‘the Master of the Vaults’. They glorify Franciscan virtues, showing Chastity as a girl at a prayer within a tower, Poverty as a marriage ceremony between the saint and his ‘Lady’, and Obedience as a friar accepting a yoke, in the presence of Prudence and Humility.

The right transept contains the gravely beautiful Madonna Enthroned among Angels with St. Francis, the work of Giotto’s master Cimabue (1240-1302). Very few frescoes by this first great name in Florentine painting survive, and this one has been much damaged over the centuries. Most striking is the way in which Cimabue obviously attempted to recapture Francis’ exact likeness from contemporary descriptions of him, right down to the thin nose and sticking-out ears.

Equally lovely are Simone Martini’s five saints, on the lower portion of the right-hand wall, studies in serene visionary abstraction. Giotto himself, if he did not paint all the vividly realized scenes on the left side of the ceiling arch, surely assisted in the overall design of each panel, and his vigorous sense of the dramatic is powerfully apparent in the two Miracles of St. Francis. One of these show a child of the Sperelli family falling form a tower, after which he is restored to life through  the saint’s intercession; the other tells the story of a boy retrieved form a house which had collapsed on top of him. The man standing on the far right is said to be a self-portrait by Giotto, and the hawk-nosed man next to him is traditionally held to be Dante (who remarked that the painter was exceedingly ugly and had six children equally hideous in appearance).

Underneath the Lower Church, in a crypt which used to be called ‘The Third Church’, lies St Francis himself. For centuries nobody knew where Fra Elia had placed his master, until in 1818, after some fifty nights of tunneling (secrecy was necessary in case someone made off with the precious relics), the sepulchral urn was discovered, walled up in travertine blocks taken from Assisi’s Roman rampart and still surrounded by a handful of the earliest offerings from the faithful. Mercifully, the neo-classical mausoleum has been removed and the plainness of unadorned stonework accords with the true Franciscan simplicity.

Where the Lower Church is characterized by candlelight and shadow and the gentle curve of wide-vaulted ceilings, the Upper Church, with its round, slit-windowed buttresses and tall clustered columns, presents you with an entirely different visual experience. The fact that you need to go out into the daylight from one to the other allows you an essential interval. Most guidebooks tell you to enter the Upper Church from the sanctuary, but though the fifteenth-century double-arcaded Cloister of Sixtus IV which you pass on your way is indeed beautiful, I recommend climbing the stairs from the porch at which you came into the Lower Church, so that you can enjoy the noble breadth of the gothic façade and get a better impression, by a view from nave to apse, of the importance of space and light in relation to the most famous of all Assisi’s fresco cycles.

This is the series of 28 scenes from the life of St. Francis, traditionally ascribed to Giotto, who probably began it in 1296, at the age of 29. His basis for these episodes was St Bonaventure’s biography of Francis (1263) derived from accounts by founder members of the order. Even if you have never seen these paintings before, you will certainly be familiar with some of them, for they are among the most popular images in the whole of Christian art. Here is The Sermon to the Birds, with its clustering finches, doves, thrushes and magpies under a bushy-branched oak tree, The Miracle of the Spring, where a poor peasant laps eagerly at the water Francis has made to flow for him, the wonderful Expulsion of Devils from Arezzo, in which bat-winged demons flit angrily off from the clustered roofs and towers of the Tuscan city, and Francis Giving his Cloak to a Poor Knight, amid a rocky Umbrian landscape with a hilltop town in the background.

Nobody questions either the significance of these frescoes in the development of Italian medieval art, or the intrinsic impact made by their combination of grandeur and human warmth, but art historians continue to rage furiously at one another over exactly who painted them. Certainly not Giotto, says one school thought, which has devised a ‘St Francis Master’ to cover the lot. Perhaps Giotto here and there, says another, unwilling to deny his claim at least to the prevailing mood of naturalistic immediacy.

The most likely answer is that, though a good deal of what we see here is by his assistants (the stiffly drawn figures in The Simpleton Honours St Francis, or some of those in The Saint Returning his Clothes to his Father, for instance), much else is by Giotto alone. The frequently made objection that the workmanship is far less sophisticated here than in the later fresco cycle in the Scrovegni Chapel at Padua takes no account of the possibilities of artistic development, for Giotto as for any other creative spirit.