Jammed on to a steep Assisi hillside and visible for miles across the valleys of Umbria, the buttressed basilica of St Francis is a masterpiece of medieval engineering and Gothic architecture. No postcard or textbook reproduction could prepare a visitor for the treasures within - frescoes which fuse artistic perfection with spirituality. Despite the relentless patter of tourist feet, there is an abiding sense of being in the presence of something doubly profound - the story of the life of St Francis and its depiction by a master.

The basilica is two churches in one, and after the contemplative darkness of the lower church and the silence of St Francis's tomb, the brightness of the upper space seems more akin to a sumptuous art gallery than a church. And as a gallery, it holds some of the great paintings in the history of Western art, including Cimabue's oxydised dynamic masterpiece of The Crucifixion and his pupil Giotto's fresco cycle of The Life of St Francis.

Giotto's involvement has been one of the longest-running controversies of Western art, with many scholars disputing whether the frescoes are by him or by pupils. In Assisi itself, he is believed to have been painting the bulk of the frescoes from 1295 onwards. And most visitors would agree that whoever painted them has a deep empathy with their subject - the most Christ-like of all the saints. Giotto, like many artists of his time, belonged to one of the lay Franciscan orders and committed himself to Francis's values. For Western art, the frescoes were a significant step away from the iconic style of Byzantine painting to the more humanist approach of the early Renaissance.

More than any other, the fresco of St Francis's Sermon to the Birds expresses both the love of natural beauty which was so typical of Francis as well as the Franciscan quality of humility, with the flock addressed as an essential part of creation. The narrative quality of the frescoes, depicting Francis's life, from his wayward youth to his discarding of wealth and commitment to a life dedicated to the poor and to Christ, and their minute detail of life in medieval Italy, were as revolutionary in their time as Francis's message.

For many followers of St Francis, the ornate basilica was an inappropriate memorial to the man who said: "Foxes have holes and birds have nests but the Son of Man has nowhere to lay his head." But until Friday morning, for this century's pilgrims - tourists, sophisticated art tour traveller or backpacker - Assisi's basilica represented the only serious rival to St Peter's in Rome as the richest and most evocative ecclesiastical collection of Italian masters of art.

Catherine Pepinster