On all these occasions Castro kept a log or roteiro, from Lisbon to Goa, from Goa to Diu, and from Goa to the Red Sea. They display a strong scientific interest, a spirit thoroughly modern—nothing, however small it might be, was to him necessarily unimportant or negligible—or perhaps ancient, since he complains that in his day the scientific investigations of the ancients were no longer in vogue. The logs are written with that vivid directness which mark his letters, “written,” he said, “not for the ladies and gallants of the Court and royal palaces, but for the mariners of Leça and Mattosinhos.”
His descriptions are precise and accurate, which does not prevent them from being often picturesque. He notices many birds, including one white and grey which, he says, the sailors call frades (monks). “I pay great attention to eclipses of the moon,” he writes, as also to longitudes and latitudes, fishes, seaweeds, currents, winds, the colour of the Red Sea, and every detail that might concern the art of navigation, to the delight of his friends Dr. Pedro Nunez and Prince Luis, who had furnished him with special instruments and other assistance for his voyage.
In the summer of 1542 he was back at Cintra, but in December of that year he was appointed to the command of the coast fleet, the main duties of which were to keep clear the coast of Portugal from pirates, such as Mondragon, who perpetually hovered in wait for the priceless spoils and cargoes of Portuguese ships homeward bound from India. He seems to have gone to sea before the end of the year and held this post for two years, with a brief interval in 1543 when he commanded the Portuguese fleet sent to co-operate with the Spanish against Barbarossa. They did not come to an engagement, and Dom João, after visiting Ceuta, returned to Portugal.
He was at Cintra in the beginning of 1545 when the unwelcome news reached him that he had been appointed Governor of India. Most unwillingly he accepted this new post, the difficulties and disquiet of which he had been able to gauge at first hand during his former sojourn in Goa. His young sons were to accompany him.
A picturesque story of the Governor-elect cannot be better told than in the words of the historian Couto, who served under him in India: “Passing one day by the door of a tailor [in Lisbon] he noticed a pair of very rich and fashionable velvet breeches, and pulling up his horse asked to see them. After examining their curious workmanship he asked whose they were. The tailor, not knowing whom he was addressing, answered that they were for a son of the Governor who was going to India. Dom João de Castro thereupon in a rage took up a pair of scissors and cut them into shreds. “Bid that young man buy arms,” he said to the tailor, and so passed on.”
At the end of March the fleet sailed. The number of men actually enlisted was eight hundred, but many more who had been rejected for some defect or were escaping from justice succeeded in embarking as stowaways. In the Governor’s ship alone there were nearly two hundred of them, and they required to be fed during a voyage of many weeks. The Governor was advised to cast them adrift in the provision ship or to maroon them in the Cape Verde Islands, but humanely and persistently refused.
He had not been long at Goa when, in April 1546, news was brought that a formidable attack was being prepared against Diu, the fort commanded by the heroic Dom João de Mascarenhas. Castro sent his son Dom Alvaro with a strong fleet to its relief. The fleet was delayed by violent storms, and when it finally reached Diu there was little of the fortress left. The walls and bulwarks were levelled with the ground, most of the defenders dead, and those who remained either wounded or ill. No one but Mascarenhas could have held on in such conditions, and even so “six more days,” wrote Castro to the King, “and relief would have come too late.”
Most of the nobles in Diu were dead, and among them Dom João de Castro’s other son, Fernando, who had been blown up with many others on a mined part of the wall on which they had rashly remained, although warned by Mascarenhas of their danger. “He should have obeyed Dom João,” wrote Castro stoically to the King, and he added: “Of what Dom Fernando did till the time of his death I will say nothing to your Highness, for it cannot be that men are so wicked but that some among them will inform your Highness of the services and great exertions that my sons undergo in your service.”
The King of Cambaya still boasted of victory, and Dom João de Castro himself sailed north with a powerful fleet from Goa. After striking terror into the enemy by ravaging the coast of Cambaya, setting it all aflame and, in his own words, “sparing no living thing,” he left these shores covered with dead and crossed to Diu.
The fortress was now again invested by an army of 60,000 Moors, and in the battle with the besieging force the Governor was himself more than once in the greatest danger before the enemy was routed. Indeed, it was his personal exertions which largely decided the day, and with pardonable pride he wrote to the King that it was “the greatest victory ever seen in all the East.”