Being a historian has its good days and its bad days. The good ones are almost intoxicatingly exciting - when you press the walls of the cave and they give way, revealing a room full of treasure that has been lying undisturbed for centuries. Those are the days when you get to feel like Indiana Jones and it all feels worthwhile.

Then there are the bad days. Forget Harrison Ford; think Charlton Heston at the end of The Planet of the Apes and his blood-curdling wail: 'What have we done ?????!!!!!'

I had a protracted spell of that when I was in Istanbul a week ago. You don't need me to tell you what a fabulous city it is, layer upon layer of gloriously rich history. The buildings, the stories, the gossip, the tragedy, the hope. It's the past in full Technicolour™

So here's the problem. As I walked through the city on the first morning, I saw a group of tourists gathered with cameras and smartphones whirring at a location I was not familiar with in the shadow of the majestic Haghia Sophia. I strolled over expectantly - new things are always being found in Istanbul: perhaps they were examining a piece of mosaic floor; or inscription; part of a column perhaps? I flicked into Indiana Jones mode. Here was something exciting; a discovery of some kind; perhaps I could even offer a few pearls of wisdom to the tourists.

They were taking pictures of a kitten.

Fair enough. Except that it was a pattern everywhere I went. By Fethiye Camii: a group of tourists were snapping a cat. By the column of Constantine: Italians taking photos of cat rolling about with another cat. At the Milion - the Hyde Park Corner of Constantinople: small tour group holding smartphones up to take shot of cat sleeping, or possibly dead. At the German Fountain: large group standing next to guide holding umbrella not listening to description of the Kaiser's visit to the city, but marveling at cat chewing on what appeared to be some kind of meat product. By the baths of Zeuxippos: twenty tourists taking photo of cat sitting on Vespa.

It was like a joke: no one seemed interested in the wonders of the city, these crown jewels of civilization. It was not once, twice or a few times, but everywhere. Ignore the rich tapestry of history that Istanbul has to offer. Come to look at its cats and kittens instead. Jeepers. Aww - look at the kitties. For 91 days.

I wandered round in something of a fog. I'm the one with the problem, I told myself. What's wrong with me: why am I more not more interested in the little furry four-legged friends; what's so great about these marvels of human creation? History, the tourists of Istanbul seemed to be saying: it's all in the past. Who cares?

You should be forgetting The Alexiad and fiendishly complicated middle Byzantine literature like the letters of Michael Psellos; read A Street Cat named Bob, which topped The Times and The Sunday Times best-seller lists. Give it's smash hit follow-up a go: Bob. No Ordinary Cat, currently shifting copies by the shedload. Forget The Odyssey by Homer - read Homer's Odyssey, the New York Times Best Seller about a man and his blind wonder cat. And then, on the flight home, it was like Groundhog Day: I opened The Times and found page three dominated by a story about: cats.

I've a few theories about all this - ranging from the decline of the west to the failure of modern education that makes learning and curiosity seem like work; from shortened attention-spans to the overwhelming opiates of fast-food, TV and pleasure-seeking. We might not have a clue about Syria, Iran, Russia, China and all those other places where it helps if you actually know something about their past and present. But we're good to go if war breaks out in the feline world: key in what does cat think into Google and you get 5 billion hits.

Cue mid-life crisis on the streets of the Queen of Cities, as Constantinople was known: why bother learning dozens of languages, acquiring different skills to handle a wide array of sources, or struggling in the small hours to make sense of the world - when what people really want to know how cats choose between chicken and liver, or whether different kinds of purring means different things ? A bit of a low point, really.

Forget Istanbul's crucial role in the new world order; forget the riots; forget Turkey's border with Syria and the acute refugee crisis; what's newsworthy as far away as India is that Barack Obama loves the cats of Istanbul.

I went to Istanbul as Indiana Jones and came back as Charlton Heston in Planet of the Cats - puzzled, perplexed and with a growing sense of doom.

A bit of a low point. Until I found something that put the fizz back into step, and got dusting off my (metaphorical) leather jacket, (imaginary) felt hat and (real) bull-whip. For that, though, you'll have to wait a few months till the book I've been working on for several years is done.

In the meantime, I understand The World According to Bob is doing brisk business: more copies sold this week than books on China, Russia and the Middle East combined.

Cats 1 Humans 0. The Apes would be proud.

I've just come back from a few days in Istanbul. As usual, trips out to the old capital of the Byzantine Empire do more than allow the chance to gawp at jaw-droppingly beautiful art or to admire monuments that in some cases are nearly two thousand years old. It is a stunning and captivating city (as it always has been). It is tempting to walk around the city as though it was a museum, taking in a 5th century bath-house here, a 10th century church there, mosques built at the height of Ottoman power peering out above the Byzantine city walls.

I wanted to use my time in the city not for wonderment, but for more practical reasons. I have been working on a text that has long been attributed to the Emperor Leo VI, who ruled Byzantium between 886-912, and acquired the nickname 'the Wise', which as far as imperial/royal nicknames go, is a good one. (I have my doubts about the attribution of the text for a number of reasons, but that's another matter.

Known as the Book of the Eparch, the text deals with the regulation of trade in Constantinople. It sets out a blue-print for how commerce worked in the Byzantine capital. Business were clustered together, with silver-smiths in one area, grocers in another; candle makers were over here; blacksmiths were over there. Traders had to be members of guilds, each with their own code of practice - and with punishments too for as and when these were breached. If you let down your fellow saddle-makers, for example, you could expect to have your head shaved and be whipped.

Trade was supervised by an official - the Eparch - who was responsible for making business run smoothly. The Eparch and his team were like Trading Standards officers: they checked that no one was being ripped off, and that transactions were fair: one of the things they cracked down on was to stop sharp traders swindling their customers.

I headed to the Grand Bazaar to see if there were any lessons I could learn that would help me understand the Book of the Eparch better - and get a closer sense of how dealers that were ultimately competing also co-operated with each other. The guild structure was great if you wanted to keep rivals out of business; but not so good if you were selling the same product as the person on the stand next door: how could you stop margins being destroyed ? Is the Book of the Prefect, in other words, one of those Byzantine texts that is more than it seems, and less than it is?

Enlightenment came quickly. The first thrill was finding the scales-shops. In a perfect echo from a thousand years ago, having accurate and certified scales was one of the key things that the Eparch and his team cracked down on. Frequent checks were made to check that scales were properly calibrated, to prevent wonky sets that enabled customers to be over-charged and let nifty dealers make tidy (and unethical) profits.

If the Eparch would have been pleased to see tamper-proof digital scales, then he'd have been satisfied too with the way that the precious metal dealers were also clustered together in a direct parallel with how they were bunched a millennium ago. This makes and made security easier (important when there are high value objects involved); it's important too because much of the silver trade now (and presumably then) is between dealers whose eye for spotting value is never sharper than when it comes to their competitors. Having dealers near each other helps rationalise prices - and that was good for the non-specialist consumer who could get second, third and fourth opinions from onlookers.

But the best lesson I learnt was from studying the layout of the many stalls in the bazaar. It occurred to me as I paced up and down that the stall-holders selling knock-off football shirts and flawless copies of Louis Vuitton suitcases function as members of guilds in all but name. They control the market - since the space in the Grand Bazaar is finite; they carry stock that ultimately comes from the same source - whether leaking out the back door of textile factories in Turkey or produced by skillful counterfeiters close by; and they all have a vested interest in getting sales without cannibalizing each others' business.

So I conducted a brief series of experiments, based on trying to see how where I said I came from, what I was wearing, and how interested I looked could condition the prices I was quoted. I set my self the task of trying to buy a Barcelona FC replica shirt - the closest parallel I could think of to the sort of foreign textiles that appear in the Book of the Eparch.

Prices swung wildly, depending on the mood, personal outlook and day's business of the stall holders: some quoted nearly treble what others suggested; opening prices were considerably lower late in the day than in the morning; saying I was from Spain produced lower price-pointing than saying I was British; in a sign of the times, the highest prices came when I said I was Russian.

And in all this, the structure of the markets in medieval Constantinople became clearer, as did the point of the Book of the Eparch. The regulation of trade was not about imperial control, about market supervision or about preventing market abuse (the three elements normally focused on by modern scholars); in fact, it's all about free trade. If you want to buy a fake Bayern Munich shirt today, you know you have to head to the Bazaar; no one forces you to buy from one stall rather than another - so you have the choice. And no one forces you to pay a set price; it's all up to you. The joy of the free market....

You have the choice too for how many stall-holders you want to bargain with, how many different traders you want to try to set off against each other to get the best deal. And therein lies the skill and the art for both seller and buyer. The dance is a painful one as my experiments showed: opening a discussion about pricing kicks off questions about my children, my jacket, my shoes, my knowledge of the country, my favourite food, whether I smoked, which music I liked. The purpose, like a spider spinning a web, is to entrap - and to do so by wearing you down: you can walk away at any time; but if you want to ask 40 stallholders how much that fake shirt will cost and to try to beat them down, it will take all day.

And at that point, equilibrium is reached: the trader's profit is balanced by the amount of energy and time the buyer is wiling to spend negotiating with multiple sellers. If the price is too high, it is worth moving on - even though that is normally the cue for the trader to restate his offer. But eventually, you reach a point where it is no longer worth squeezing the price lower and you succumb. Everyone wins - the stallholder gets his cash with whatever the profit is; the buyer gets his shirt; and the next door trader sits and waits for the next punter to fly past.

I've always thought the threat of shaving and flogging errant traders was a little unrealistic and unreasonable - not least since as anyone who's visited the Grand Bazaar will tell you, those traders have the gift of the gab. Lovely jubbly, the lucky man said, as he wrapped a Barcelona shirt into a bag for me. I'm still mulling what the equivalent would have been in 10th century Constantinople....

I've been working recently on attitudes to Muslims in Europe after the fall of Constantinople to the Turks in 1453; I'm particularly interested in how English views contrasted sharply with those of other places in the Mediterranean - especially during the reign of Elizabeth I. The Virgin Queen was so friendly with Muslim potentates that one modern scholar has accused her (and England) of 'participating in a jihad' against Christian Spain. So of course, one of the first places to look is Shakespeare, who provides such a wonderful lens to look at this period.

Amongst the plays I've been reading to get a sense of what ideas about immigration were like in the early 1600 is Othello. Last night, I was lucky enough to see an electrifying performance at the National Theatre, with Adrian Lester in the title role and Rory Kinnear playing Iago. The reviews have been ecstatic. Quite right: at the climax, there was not a person in the house who did not have hairs standing up on the back of their necks.

Although set in Venice and Cyprus, the play tells us much about contemporary views in England about foreigners. What stands out is not how Islam was seen (in fact, there are only passing references), but how Shakespeare sets out to attack the views, policies and behaviour a group that is instantly recognisable today: anti-immigration, isolationist, unreconstructed, pub-going men (no change there then) who believe the country is better off without foreigners coming in from abroad to take the best jobs: UKIP voters.

It's 'the curse of service' says Iago at the start, to have to serve as Othello's lieutenant - awful to have to work for a foreigner;  better to be his hangman, smirks one of his friends. We don't want his sort around here, they agree, slapping each other's backs.

So does Desdemona's father, whom Iago and his friend resolve to 'poison his delight', 'plague him with flies' and shame him in the streets by shouting that his daughter had married a foreigner - who right now, they tell the old man gleefully, is 'making the beast with two backs' with his daughter; an 'old black ram is topping your white ewe' as we speak, they say.

The shame of his daughter marrying a foreigner kills Desdemona's father, as we learn later in the play: 'thy match was mortal to him and pure grief/Shore his old thread in twain', she is told. Rather than pride at a son-in-law commanding the armed forces, the father was broken hearted by the fact that he was from abroad (Othello's skin colour would have been as shocking in Tudor times as in 1950s America)

Iago and his friend Rodrigo can barely bring themselves to acknowledge what is blindingly obvious to them: foreigners; they take the jobs and marry the girls. If there had been a welfare state in Tudor times, they'd have accused Othello of defrauding that too - scrounging benefits, getting hospital treatment on the cheap and more besides. Othello's rise to the top is resented by people who feel more should be done for the local population than these newcomers - regardless of talent.

Shakespeare makes mincemeat of these narrow-minded, jealous and selfish little Englanders, who put their closeted views and ideas of personal gain ahead of what is good for the country: ultimately, Venice loses one of its great commanders - who knew better than anyone how to defeat a powerful and resourceful enemy. Driven into a frenzy of jealousy, [spoiler alert!], Othello kills his wife and then himself. The jealousy and individual advancement comes before the common good of the state; there are no winners.

But this of course is what all politics is about: you can't win. What prevails are the mind-games, manipulation and Machiavellian scheming. Innuendo, leaks, spin-doctoring and briefing against rivals, carried out from under the veil of the ends justifying the means. The appalling slurs on politicians' parents, their backgrounds, their private lives are par for the course in a world that has lost its way, where the individual counts for everything. The Miliband fiasco this week is one example; Damien McBride's new book out this week: Iago & Proud would have been a good sub-title.

But it is explicit too. Women would not be stupid enough to abandon their convictions for money, power or advancement, Desdemona says. Some might, says her confidante, Emilia: after all, look at how men have messed up the economy, how widespread domestic violence is, and find the concept of staying faithful like a 'sport'. It is hard to disagree.

So there we go: Shakespeare, who as always is right about everything, has the answer for contemporary voters and politicians alike: don't vote UKIP; and get more women in politics. I could not agree more.

Now. Back to Islam in the Mediterranean in the 16th century.