The Crusaders spread new ideas on art and architecture. Paul Barker travels to Gubbio to find out how they were developed by the Italians

On the bright, hot stone of the hill-edge piazza a mongrel collie races around after a yellow football. It dribbles brilliantly. Standing next to grandiose Renaissance palaces at either end of the square, everyone laughs and applauds. Two boys, the dog's owners, cycle around in interweaving circles. There's a crucial Milan-Turin match tonight. For us all, the dog is a warm-up for the TV football screening which will later fill all the bars.

This is the Piazza Grande, artistic high point of the small north-Umbria hill-town of Gubbio, about 25 miles from Perugia. You look out over ochre roofs towards green hills (where we're staying in a paradisal guest-house). The high, flat, white frontage of the stupendous Palazzo dei Consoli bangs the sunlight straight back into the piazza and on to the dog, like the reflector in a fashion photo-shoot. At the palace's top left corner a turret is perched so awkwardly you think it will fall off (though it's stayed affixed for five centuries so far). At the other corner is an iron cage where condemned criminals had to repent in public. (Now there's an idea.)

Gubbio, like many Umbrian towns, is a symphony in stone. Far below the piazza (you can take a lift) a long, dark street of once-grand houses winds along. Many have bricked-up doorways, their function long forgotten. Some say they were only unbricked to let coffins in and out. In shadow, the street looks mournful, sepulchral. Then the sun shifts and brings the stone to life.

Nothing happened here for centuries, after Gubbio's Renaissance heyday. There's still a frontier-town, backwater feel. Federico, the great 15th-century duke of nearby Urbino, built a back-up ducal palace here, opposite the cathedral. On one day in May, wax images are toted high up the hill, to the church of Sant'Ubaldo. Before Pasteur invented immunisation, your best bet for curing rabies was a quick prayer to Ubaldo. There's something about Gubbio and canines. Way back, St Francis was drafted in to quieten a rumbustious wolf.

The museum has a set of pitch-black tablets covered in spiky characters, cherished as rare evidence of the ancient Umbrian language. Everything went in threes: three gods, three phases to each sacrificial ritual, three animals per god, three beneficiaries per sacrifice.

It's market day. Solid ladies go along with plastic bags to a lower town square named after the "Forty Martyrs": innocent civilians shot by the Germans in the Second World War in reprisal for raids by partisans from the hills. But, in Italy, history isn't a slate to be wiped clean. It's a palimpsest, where you write message on top of message, century after century, year after year. So, the main monument in a square named after anti-fascists is, paradoxically, a fascist one. A tall, art-deco war memorial, surrounded today by market boxes of peppers and vases of gladioli. Its bronze soldier commemorates Gubbio's "glorious dead" from one of Mussolini's North African colonialist wars in the 1920s.

In the church of San Francesco, behind the market stalls, is evidence of other, earlier militarism. Within the cool nave a farmer takes off his brown trilby to pray. The plain columns rise up above him into a curved roof which looks exactly like a mosque. The Crusaders did not just bring loot from Byzantium and the Levant. They brought back new ideas of art and architecture.

Apart from its own delights, Gubbio is at the heart of a wondrous network of towns and villages where you can track how Italians took these ideas forward. Our own scheme was for one day's idling or reading, then one day's exploring. Take Dante's Inferno with you: it evokes the vivid, vengeful world the new art sprang from.

To my surprise, Assisi, 45 minutes south of Gubbio, is pleasant - and somehow moving, slightly off-season at least, in spite of the avalanche of pilgrims. In the vast basilica of St Francis, a Madonna by Cimabue is as laden with gold leaf as anything in Byzantium. Then Giotto, restored after the 1997 earthquake, tops it with his stupendous strip-cartoon of the saint's life. When St Francis is preaching to the Pope, you can see the thought behind the pontiff's bleak face: "Shall I listen to this man, or just have him burnt?" The basilica, by the way, is not Assisi's cathedral. This is in a back street and is often missed; it's awash with stone pelicans, griffins and other fabulous beasts.

Assisi's stone is pink. Urbino, an hour north-east of Gubbio, is a cantata in beige. With a vigorous university, it's no mere museum town. It has a pleasant tendency to break out into bicycle races or kite festivals. When we were scrutinising the Renaissance wonders in Federico's No 1 palace, a tremendous racket drew us to the window. A rally of 1950s cars was roaring up the ancient streets. I thought Michael Caine should be in there somewhere.

In the duke's palace, with its two 15th-century pictures by Piero della Francesca, we decided to hit the "Piero trail". His paintings have an unparalleled ability to combine geometry and poetry, and many are here in his native terrain. The National Gallery has a couple, too.

You begin in San Sepolcro, a short drive from Gubbio, where Piero lived most of his life. His Resurrection, here, is overwhelming evidence of the power of art - and of journalism. A British officer was ordered to shell this small town. But he remembered an essay by Aldous Huxley, arguing that this was the greatest painting in the world. He held his fire, hoping the Germans would withdraw. They did. Half a century later we sit in front of this same picture, rapt and, amazingly, alone.

We're also alone in front of the stunning Madonna del Parto (a pained-looking Mary in the ninth month of pregnancy) in the pretty village of Monterchi. Now, boost your strengthwith fresh bruschetta in the delightful delicatessen next door - for the next stop is Arezzo. Here, when you've seen the extraordinary frescos of The Legend of the True Cross, you've been in the glorious presence of more Piero della Francescas than anywhere in the world. There is no substitute. As we leave, trumpeters, looking like fresco characters, are practising the Star Wars theme in Arezzo's stony arcades.

Nor is this the half of it. Spello (more pink stone) is one of many delectable hill towns and villages nearby. It's a town of labyrinthine alleys, and also has a statue of St Peter who looks fairly fraught, thanks to the shiny meat cleaver embedded in his head. After that, it's time to take refuge in a pistachio ice or a glass of Umbria's excellent white wine.

Ryanair (0871 246 0000; offers flights from London to Perugia and Ancona. Return fares start at £50. Perugia is around one hour's drive from Gubbio and Ancona two hours. Contact the Italian State Tourist Board on 020-7408 1254 and see