The 16th century saw the development of the man-of-war, a truly ocean-going warship, carrying square-rigged sails that permitted tacking into the wind, and heavily armed with cannons. The adoption of heavy guns necessitated their being mounted lower down than on top of the fore and after castles as previously where anti-personnel weapons had been positioned through the later Middle Ages, due to the possibility of capsizing. This meant that what had earlier been the hold of a ship that could be used either as a merchant ship or warship was now full with cannon and ammunition. Hence ships became specialised as warships, which would lead to a standing fleet instead of one based on placing temporary contracts.