Was Caligula as mad and bad as he's portrayed? The jury's still out.

The excellent and entertaining historian Mary Beard's BBC 2 documentary on the bad (possibly), but not necessarily mad Roman Emperor Caligula was great TV, despite the paucity of evidence available to her (or anyone else). Ms Beard brought all her academic learning, mild eccentricity and jaunty presenting skills to bear as she sought to rehabilitate the Emperor with the worst reputation of any of the Julio-Claudians, and in a list that includes Nero that's not bad going.

But did she bring us anything new? Unfortunately, I think not. Basically all that impressive insight and interpretation is speculative theory and almost all of it has been said before.

Of course Caligula's reputation suffered at the hands of the ancient historians, and the modern entertainment industry has capitalised on it. Any contemporary sources have been lost and the histories were written by men with their own political axes to grind sixty, eighty and a hundred and fifty years after the event. Tacitus is the Daily Mail of Roman historians, always interesting, but the hidden, and often not quite so hidden agenda, is always there. Suetonius obviously has his own sources, but he also picks and chooses from Tacitus, sometimes adds in an extra detail, a claim or an allegation, and lays it out like an old-fashioned Times, the newspaper of record, though the record is infuriatingly devoid of a timeline. Dio Cassius (think the late News of the World), cherrypicks from both, embellishes with a twist of spice (A squadron of war elephants invading Britain with Claudius? Please!) and a little outrageous, usually unattributed, speculation. It's only by reading between the lines and comparing them that you have a chance of getting anything like a balanced portrait, and even that's skewed by other factors.

My take on Little Boots?

Did Caligula make his horse Incitatus a consul? Of course not. He's not on the list for a start, but equally, Caligula as Emperor had an enormous respect for the institutions of Rome. He may have said it as a joke or a threat, but I'm certain it didn't happen. Mad or not, he may have had a sense of humour.

Did he spend countless millions to build a bridge of boats from Baiae to Puetoli in the Bay of Naples to outdo Poseidon? Probably. Who could make up something as daft as that and there's enough detail about the ships and the scale of the event to believe it's true.

Did he sleep with his sister? I think the jury's out. They lived in a different sexual world and Caligula had been brought up on Capri in the court of Tiberius where the Emperor seems to have gone to any lengths in an attempt to rekindle his waning sexual prowess. His social norms are unlikely to have been the same as ours.

Did he have thousands of people, mostly innocent, killed in horrible ways? The numbers are up for debate, but there's plenty of individual detail in all the histories that has the ring of truth. Why would Suetonius devote so many lines to the horrific, but undoubtedly fascinating death of a lowly animal dealer with his hand in the till (it's all in my scintillating debut novel) unless it happened? The other side of the coin is that Suetonius tells us that Caligula's kindly uncle Claudius had just as many executed or assassinated, at the instigation of his greedy and manipulative band of freedmen.

It's an undoubted cliche that history is written by the victors, but after researching six Emperors for seven Roman novels I find the view difficult to argue with. Historians can only take the evidence, both physical and written, and use a mixture of learning, common sense and, ideally, a healthy dose of scepticism, to interpret them. Anything else is dangerously close to rewriting history. Fortunately, Mary Beard managed to stay on the right side of the line.