Robert Fisk: The US forces, like the Crusaders before them, are prisoners in their own fortresses

Sitting in Saddam Hussein's palace they can stare over the parapets but that is as much as most will ever see of Iraq

I drove Pat and Alice Carey up the coast of Lebanon this week to look at some castles. Pat is a builder from County Wicklow, brave enough to take a holiday with his wife in Beirut when all others are thinking of running away. But I wanted to know what he thought of 12th-century construction work.

I drove Pat and Alice Carey up the coast of Lebanon this week to look at some castles. Pat is a builder from County Wicklow, brave enough to take a holiday with his wife in Beirut when all others are thinking of running away. But I wanted to know what he thought of 12th-century construction work.

How did he rate a Crusader keep? The most beautiful of Lebanon's castles is the smallest, a dinky-toy palisade on an outcrop of rock near the village of Batroun. You have to climb a set of well-polished steps - no hand-rails, for this is Lebanon - up the sheer side of Mseilha castle and then clamber over doorsills into the dark, damp interior.

So we padded around the battlements for half an hour. "Strongly made or they wouldn't be still here," Pat remarked. "But you wouldn't find any company ready to put up the insurance. And in winter, it must have been very, very cold."

And after some minutes, he looked at me with some intensity. "It's like being in a prison," he said.

And he was right. The only view of the outside world was through the archers' loopholes in the walls. Inside was darkness. The bright world outside was cut off by the castle defences. I could just see the splashing river to the south of the castle and, on the distant horizon, a mountainside. That was all the defenders - Crusaders or Mamlukes - would have seen. It was the only contact they had with the land they were occupying.

Up at Tripoli is Lebanon's biggest keep, the massive Castle of St Gilles that still towers ominously over the port city with its delicate minarets and mass of concrete hovels. Two shell holes - remnants of Lebanon's 1975-1990 civil war - have been smashed into the walls, but the interior of the castle is a world of its own; a world, that is, of stables and eating halls and dungeons. It was empty - the tourists have almost all fled Lebanon - and we felt the oppressive isolation of this terrible place.

Pat knew his Crusader castles. "When you besieged them, the only way to get inside was by pushing timber under the foundations and setting fire to the wood. When they turned to ash, the walls came tumbling down. The defenders didn't throw boiling oil from the ramparts. They threw sand on to the attackers. The sand would get inside their armour and start to burn them until they were in too much pain to fight. But it's the same thing here in Tripoli as in the little castle. You can hardly see the city through the arrow slits. It's another - bigger - prison."

And so I sat on the cold stone floor and stared through a loophole and, sure enough, I could see only a single minaret and a few square metres of roadway. I was in darkness. Just as the Crusaders who built this fortress must have been in darkness.

Indeed, Raymond de Saint-Gilles spent years besieging the city, looking down in anger from his great fortress, built on the "Pilgrim's Mountain", at the stout burghers of Tripoli who were constantly re-supplied by boat from Egypt. Raymond himself died in the castle, facing the city he dreamed of capturing but could not live to enter.

And of course, far to the east, in the ancient land of Mesopotamia, there stand today equally stout if less aesthetic barricades around another great occupying army. The castles of the Americans are made of pre-stressed concrete and steel but they serve the same purpose and doom those who built them to live in prisons.

From the "Green Zone" in the centre of Baghdad, the US authorities and their Iraqi satellites can see little of the city and country they claim to govern. Sleeping around the gloomy republican palace of Saddam Hussein, they can stare over the parapets or peek through the machine-gun embrasures on the perimeter wall - but that is as much as most will ever see of Iraq.

The Tigris river is almost as invisible as that stream sloshing past the castle of Mseilha. The British embassy inside the "Green Zone" flies its diplomats into Baghdad airport, airlifts them by helicopter into the fortress - and there they sit until recalled to London.

Indeed, the Crusaders in Lebanon - men with thunderous names like Tancred and Bohemond and Baldwin - used a system of control remarkably similar to the US Marines and the 82nd Airborne. They positioned their castles at a day's ride - or a day's sailing down the coast in the case of Lebanon - from each other, venturing forth only to travel between their keeps.

And then out of the east, from Syria and also from the Caliphate of Baghdad and from Persia came the "hashashin", the "Assassins" - the Crusaders brought the word back to Europe - who turned the Shia faith into an extremist doctrine, regarding assassination of their enemies as a religious duty.

Anyone who doubts the relevance of these "foreign fighters" to present-day Iraq should read the history of ancient Tripoli by that redoubtable Lebanese-Armenian historian Nina Jidejian, which covers the period of the Assassins and was published at the height of the Lebanese civil war.

"It was believed that the terrorists partook of hashish to induce ecstatic visions of paradise before setting out to perform their sacred duty and to face martyrdom..." she writes. "The arrival of the Crusaders had added to ... latent discontent and created a favourable terrain for their activities." Ouch.

One of the Assassins' first victims was the Count of Montferrat, leader of the Third Crusade who had besieged Acre in 1191 - "Saint Jean d'Acre" to the Christians - and who met his death at the hands of men sent by the Persian "terrorist" leader, Hassan-i Sabbah. The Assassins treated Saladin's Muslim army with equal scorn - they made two attempts to murder him - and within 100 years had set up their own castles around Tripoli. They established a "mother fortress" from which - and here I quote a 13th-century Arab geographer - "the Assassins chosen are sent out thence to all countries and lands to slay kings and great men".

And so it is not so hard, in the dank hallways of the Castle of St Gilles to see the folly of America's occupation of Iraq. Cut off from the people they rule, squeezed into their fortresses, under constant attack from "foreign fighters", the Crusaders' dreams were destroyed.

Sitting behind that loophole in the castle at Tripoli, I could even see new meaning in Osama bin Laden's constant reference to the Americans as "the Crusader armies". The Crusades, too, were founded on a neo-conservative theology. The knights were going to protect the Christians of the Holy Land; they were going to "liberate" Jerusalem - "Mission Accomplished" - and ended up taking the spoils of the Levant, creating petty kingdoms which they claimed to control, living fearfully behind their stone defences. Their Arab opponents of the time did indeed possess a weapon of mass destruction for the Crusaders. It was called Islam.

"You can see why the Crusaders couldn't last here," Pat said as we walked out of the huge gateway of the Castle of Saint Gilles. "I wonder if they even knew who they were fighting."

I just resisted asking him if he'd come along on my next trip to Baghdad, so I could hear part two of the builder's wisdom.