Fort Jesus, the Portuguese-built stronghold that stands sentinel over the gently shimmering waters of Mombasa harbour with their clustered dhows, first drew me to Portugal's history. As a boy I lived on a farm in western Kenya. Occasionally, when the price of maize was favourable or coffee had had a particularly good year, my father would treat my mother, my sister and me to a holiday on the coast. There, each time, would be the fort – a quietly brooding monument to Mombasa's turbulent past and to the seemingly mysterious role played in it by the Portuguese.

In Oxford, on a cool November day of 1960, my interest in Fort Jesus was unexpectedly re-kindled. Gazing at a display in Blackwell's window, I caught sight of a new book with a bright, glossy dust jacket: Fort Jesus and the Portuguese in Mombasa by C. R. Boxer and Carlos de Azevedo. There, splashed across the front, was a photograph of the fort, its mellowed seawalls supporting their serrated Arab battlements, fringed by gently swaying coconut palms, with the little beach nearby. As soon as I could, I scraped together from my modest student allowance the required twenty-one shillings and bought that book. It was the fort, and the book about the fort, that led me in due course to write A History of Portugal and the Portuguese Empire: From Beginnings to 1807.

One of the first European kingdoms to establish stable borders, Portugal has had an unusually long and rich history.