That the Grand Master of the Order of St John, Hughues Loubenx de Verdalle, showed utmost interest in what the protagonist of this paper had to say about the Ottoman navy, logistics, and the state of defense of the strongholds in the Eastern Mediterranean does not come as a surprise. The diary of the pharmacist from Königsberg Reinhold Lubenau is one of the very few authentic documents of a Christian eyewitness participating in an Ottoman naval campaign. Lubenau’s notes on his sojourn in Constantinople (1587–1588) have been investigated by researchers of Turkish culture, art, folklore, and politics, however his subsequent report of his participation in Hasan Pasha’s naval campaign through the entire Mediterranean have never been studied by historians. His detailed notes contain substantial information on Ottoman military proceedings and intelligence. They also give the reader an insight on how the Ottomans integrated their North African satellites—Tunis, Tripolis, and Algiers—in their bigger scheme of controlling the Eastern and Central Mediterranean and their fight against the Hapsburg Empire. It is also a most interesting document in how far the Ottoman military machine benefitted from the fresh input of occidental specialists, and military techniques. This includes the phenomenon of the ‘renegades’, men from the German lands, England, Italy, or Spain who converted to the Islam and hoped to make a career in the Ottoman army. This paper intends to analyze the contents of Lubenau’s campaign diary in light of the situation of the Ottoman fleet and military strategy at the time.