They've just discovered a bit of Beirut's Crusader castle. Most of the other great coastal cities of Lebanon – Tripoli, Batroun, Jbeil (for which read Byblos) and Sidon – have their castles in various states of perfect preservation or decay, but the Ottomans and then the Lebanese commercial elite managed to destroy the great keep that guarded the port of Beirut, the first to use as a quarry for their own walls, the latter out of greed.

After all, who wants a bloody great Crusader castle blocking the road to a thriving new port? So "zap" went Beirut's majestic castle. You can see it now only in the sketches of W H Bartlett in 1838 and of Samuel Green and – already in a pathetic state of disrepair – at the bottom of a British war department map of 1857.

Until, that is, Hans Curvers, one of the archaeologists who have dug through the underworld of Beirut and who is no helping, along with urban planner Amira Solh, to create a "heritage trail" – yes, aaagh, I hate the word "heritage" as much as any reader, of which more later – came across the very lowest wall of the castle. He shows it off proudly and, for a moment, I rather thought this Belgian-German archaeologist would like to claim ownership. The beautifully dressed Crusader stones – each with a delicately cut rim – are the base of a tower, the westernmost limit of the Crusader castle. The Ottomans built their own fortress on top of it and part of their tower exists too.

And, of course, you can find Crusader stones in the Ottoman fortification, just as you can find Roman columns embedded for support within the Crusader remains and – can we be surprised? – the 19th-century Brits, when they arrived in Beirut to defend the Druze from the Maronites (the French had already arrived to defend the Christians), built their own barracks on top of other defences. In other words, each new military force, Roman, Omayad, Turkish, British, the Lebanese themselves, physically used their predecessor's history. Walking through the ruins with Hans, you can, far below the level of the forthcoming "trail" – outside the dry moat of the lost castle – see the relics of a Canaanite wall. In other words, the old city of Beirut is a giant historical club sandwich, the lower slice of stone "bread" being about 5,000 years old.

Like archive photographs, therefore, ancient cities are about memory – not so much about loss, but about witness. The Romans were here. The Crusaders were here, and then the Muslims came. Indeed, one of the most beautiful mosques in Beirut, less than half a mile away, was a Crusader church, and when you go inside, it clearly was a Christian place of worship, complete with medieval arched windows and apse. And when the French mandate authorities built their own shopping streets, they often used real Roman marble columns on either side of their doors to prettify their buildings. Then came the 1975-90 civil war to wreck the lot. Most of mandate Beirut – though not the Ottoman bit, which was also trashed – has been restored. But the French street names have remained. So the titans of my Dad's First World War – Foch, Clemenceau, Allenby, Weygand – still grace the walls.

Weygand – the man who dismissed Churchill's 1940 offer of common citizenship as "fusion with a corpse" and initially joined Vichy – has survived, but the hero of Verdun did not. When the British, Free French and Australians invaded Lebanon, they wouldn't tolerate "rue Maréchal Pétain" any more. As Hans discovered, it was appropriately changed to "rue de France", a good idea since Lebanon was ruled by Pétain's Vichy for more than a year, complete with Nazi anti-Jewish laws. And, of course, wartime tourists thus included visitors from the Gestapo. But, since we are talking history, it should be recorded that this racist legislation was vigorously condemned at the time both by the Sunni Mufti of Beirut and the Maronite patriarch.

Hans, archaeologist and consultant to Solidere – which owns much of the centre of the city and continues to build there – is the giant behind the magical mystery tour, pointing to a Persian gate complex at one moment, then blithely leading the way past Canaanite buttresses, a bullet-pocked wall (from the 1975-90 smiting), a Hellenistic sacred well, and a mass of late Roman straw grinders. Angus Gavin, the head of Solidere's urban development, describes Beirut as the "multilayered city par excellence"; in fact, the entire pre-1830 city lies in the Solidere area. A site museum would be included in the tourist walk; indeed, the whole trail would itself be a museum.

But I'm worried about the "heritage" bit, with its corny linguistic inheritance. There might be a site museum on a 5,000-year old Canaanite wall, but no one's going to dress up in Roman uniform or Crusader armour. It's true that there have been so many political assassinations in Beirut that one group of students has also cruelly suggested an "assassination trail" for tourists. I think "history trail" might be better for Solidere. Even a "memory trail" – but which of Lebanon's religious communities will then try to lay claim to the largest memory?

And there's one other problem – and I'm setting aside a main road with Lebanon's homicidal drivers that bisects the two-mile trail – and that's the Berri militia. To be fair, it's a car park of trucks belonging to Beirut's parliament police force whose boss is the Shia speaker, Nabih Berri. He is one of Lebanon's smartest politicians – he also once helped to save my life during the civil war – but he really should move his lads off the site, which contains another part of Roman-Persian Beirut. The only alternative – an exclusively Fisk idea, I have to admit – is to incorporate these armed men as antiquities themselves, so that tourists can take pictures of their clapped-out vehicles and exotic grey camouflage uniforms and poppy-red hats. Then you really could use the word "heritage".