Yet historicism has its limits. In the four castles cited, the extraordinary workmanship, though admirable, tends to overwhelm. The mind is oppressed by detail, which does not ring true to the age it seeks to emulate.

Much more freedom is to be had from ruins, and here the choice is vast. Again a good starting-point is Wales, this time the north, which, along with western Syria, has the densest concentration of great castles in the world. In the 50-odd miles between the Menai Strait and the River Dee are two fortresses with walled towns attached, Caernarfon and Conwy, the concentric Beaumaris, necessary defence for its flat and marshy site, the twin gatehouses of Rhuddlan, and the majestic circular donjon of Flint. To these must be added Harlech, whose imposing gatehouse performs the role of the traditional keep. The impetus for this superb building programme, carried out by Master James St George of Savoy, was Edward I's conquest of Wales.

Edward's reign (1272-1307) marks the apogee of castle building in these islands, which makes North Wales a must for the enthusiast. Attention may then turn to France, whence the stone castle as we know it in Britain originates. With so much on offer, it is perhaps invidious to be specific; I would simply mention that Richard I's Château Gaillard, built above the Seine in the closing years of the 12th century, the striped walls and towers of Angers on the Maine and the precariously perched Cathar strongholds in the Pyrenees will not disappoint. Further afield, Spain has fine castles built along the Rivers Douro and Tagus as part of the Reconquista, and the enormous Moorish fortresses of Málaga and Almería on the Mediterranean. Southern Italy was the stage for a castle-builder to rival Edward I, the Hohenstaufen emperor Frederick II, known to his contemporaries as Stupor Mundi, whose masterpiece is the Castel del Monte in Apulia. In Poland, near the Baltic, there is the colossal brick fortress of the Teutonic Knights at Marbork (Marienburg).

These are just some of the highlights of Europe's astonishingly rich legacy from feudal times, enough to last a castle-hunter for a lifetime. But it would be surprising during such forays if interest was not awakened in Syria, from where Crusaders brought back so many new ideas about military architecture. It is here that the Hospitallers and Templars stamped their presence on the Levant by building what are considered to be the finest castles of all times.

At their apex stands Krak des Chevaliers, a 12th-century Hospitaller stronghold. Its dramatic location, guarding the gap between Homs and the Mediterranean, the sheer size of its walls and towers, the ingenuity of its concentric design, the warmth of its stone and its vaulted great hall and chapel leave an overwhelming impression of strength and grace. The art historian T. S. R. Boase described it as follows: "As the Parthenon is to Greek temples and Chartres to Gothic cathedrals, so is the Krak des Chevaliers to medieval castles, the supreme example, one of the great buildings of all times."

Clearly visible from Krak is the massive rectangular keep of Safita. Nearly 90ft high, it was built by the Templars, who had their headquarters at Tartus on the Mediterranean a few miles away. Unusually, its ground floor is taken up by a chapel, which today serves as a Greek Orthodox Church. Above is a nobly vaulted chamber for the garrison.