To the north of Tartus, also on the coast, is Marqab, another Hospitaller masterpiece, most of its walls as black as the volcanic rock on which it stands. In his book, Monuments of Syria (I B Tauris, 1999), Ross Burns writes of its builders: "They employed new concepts in military fortification still only tentatively being exploited in Europe, and exceeded them in scale and boldness."

This brief survey of Syrian fortifications would not be complete without mentioning three other categories. The first is Sahyun, which was built by neither of the knightly orders. Its castle stretches more than 800 yards along a ridge between two ravines, with distant views of the Mediterranean. Its most astonishing feature is a rock-cut ditch guarding the eastern approach. The ditch measures 90ft deep by more than 65ft wide and in the middle of it stands a great stalagmite of rock, left in place to support the now vanished drawbridge. Despite these precautions, the castle fell to Saladin in 1188 and was never recovered.

Into the second category fall two Arab citadels: that in Aleppo stands on an oval hill, guarded by a monumental gateway on its southern side and surrounded by a glacis and a ditch. The one in Bosra, near the Jordanian border, encompasses within its vast walls and towers a Roman theatre with room for between 8,000 and 9,000 spectators.

Finally, to the east there are two magnificent fortresses built in the sixth century under the Byzantine emperor Justinian. The crumbling walls of Resafe are in the desert and enclose a notable church and gigantic cisterns. Those of Halabiye, thrown up by the cold, green waters of the Euphrates, are in the form of a triangle, at its apex a keep made of huge gypsum blocks.

This lightning tour through the castles of Europe and the Levant has led from the opulence of 19th-century reconstructions to stark ruins open to the wind and rain, with all the excitement that that brings in the discovery of dungeons, spiral staircases, garderobes, sally ports, earthworks, wells and the remnants of Gothic vaults. But there is still a stage to go.

I have long been haunted by photographs in Guillaume Janneau's book on French military architecture, L'Architecture militaire en France (Editions Garnier, 1979), of the Château de Coucy in northern France. They show a huge cylindrical keep, measuring nearly 210ft from the bottom of the moat to its roof, dominating a curtain wall strengthened by four circular towers. The castle, which dates from the 13th century, was occupied by the Germans during the First World War and largely destroyed by them in 1917, an act which led to its being declared by the French as "a memorial to barbarity".

Today, Coucy is a sad stump of its former self. But at least some of it survives above ground. The same cannot be said of Queenborough Castle, whose description in Professor Brown's book English Castles (Boydell Press, 2004) has likewise haunted me. The fascination of this former fortress on the Isle of Sheppey in Kent lies both in its radical design and its complete disappearance.