The crews had been brought up to full strength by enlisting mule-drivers, field-labourers, and other inexperienced landsmen, and would have been better for training at sea; but except for some drills on the landlocked waters they were left in idleness, and sickness soon broke out among them and thinned their numbers. The ships thus inefficiently manned presented a formidable array. There were some five hundred in all, including, however, a number of large merchantmen hastily fitted for war service. Just as modern men-of-war are provided with steel nets hanging on booms as a defence against torpedoes, so it would seem that some at least of Antony's ships had been fitted with a clumsy device for defending them against attack by ramming. Below the level of the oars, balks of timber were propped out from their sides at the water-line, and it was hoped that these barricades would break the full force of an enemy's "beak." But the invention had the drawback of diminishing the speed of the ship, and making quick turning more difficult, and thus it increased the very danger it was intended to avert. Another feature of the larger ships, some of them the biggest that had yet been built for the line of battle, the "Dreadnoughts" of their day, was that wooden castles or towers had been erected on their upper decks, and on these structures were mounted various specimens of a rude primitive substitute for artillery, ballistæ, catapults, and the like, engines for discharging by mechanical means huge darts or heavy stones. These same towers were also to be the places from which the Eastern bowmen, the best archers of the ancient world, would shower their arrows on a hostile fleet. But locked up in the bottle-necked Ambracian Gulf the great fleet, with its tower-crowned array of floating giants, had as little effect on the opening phase of the campaign as if its units had been so many castles on the shore. Agrippa soon felt that there was no serious risk of any attempt being made by Antony to interrupt the long and delicate operation of ferrying over an army of a hundred thousand men and some twelve thousand cavalry from Italy to the opposite shore of the Adriatic. He took the precaution of watching the outlet of the Ambracian Gulf with his swiftest ships. The narrow entrance, while making it difficult to force a way into the Gulf, had the disadvantage of all such positions, that a large fleet would take a considerable time to issue from it into the open sea, and it was therefore comparatively easy to blockade and observe it. If Antony showed any sign of coming out, there would be time to bring up the whole fleet of Octavian to meet him in the open.