Wealth from trade was the mainspring of Western material advance; the visible agents of change were great guns. These came of age in Europe in the 15th century. On land their potency in reducing castle walls favoured central over local power, since in general only monarchs could afford siege-trains; so nation-states were consolidated and extended into great territorial empires. At sea, guns transformed sailing ships into mobile castles virtually impregnable to opponents who lacked equally powerful ordnance. With the ocean-going gunned warship, western Europe began to extend around the globe.