It has minarets.Huge thudding mistakes and discrepancies abound in the Latin phrases (one recurring example - the senior Roman government official in the city lives in the villa praefectus).
And the first century city has minarets.The presentation of characters' names is horrendously inconsistent - some are Latinised, some Grecianised, some Hebrew (or possibly Yiddish), and one who is called "Fabulous" (sic).
And he seems to think that there were minarets in the city before the Turkish conquest of 1453, and six centuries before the foundation of Islam.Even the transcription of the opening of St Mark's Gospel in Greek is incorrect, which is pretty astonishing as all you have to do is find a copy of Nestlé-Aland - I've got one I can lend you if you like. But (as you may have noticed) I keep coming back to the minarets; it's only one word in one of the book's rare descriptive passages, but it demonstrates the utter superficiality of the author's research into the historical setting.
The train-wreck of the author's attempts at world-building made it difficult to absorb the actual plot, but I did my best. It is set between the first and second scenes of The Romans - it turns out that the Tardis falls off a cliff near Byzantium and the Doctor, Ian, Barbara and Vicki all get separated when they get swept up in a riot in the city. A thinly contrived sequence keeps them separated until the end of the book when they discover the Tardis has been taken to Italy; in the meantime the Doctor has helped the local Christians write the Gospel of St Mark. Topping writes Barbara rather well, Ian very badly, and the Doctor and Vicki tolerably. (There is a framing narrative with Ian and Barbara, now married in 1973, taking their son to a museum where they see Ian's old sword.) The most memorable of the supporting characters are some nymphomaniac Roman ladies, and that is not saying much.
I am having difficulty deciding whether or not this is the worst Doctor Who book I have read. The only ones that approach it in awfulness are Eric Saward's novelisation of The Twin Dilemma and Topping's Telos novella Ghost Ship. In the end I think Byzantium! takes the prize for sheer quantity of awfulness; it is roughly twice as long as the other two combined. I will send my copy to the first person who asks nicely; I have no interest whatsoever in keeping this book in my collection.