The classic cog, with a rear-mounted rudder, is first mentioned in 948 AD. These early cogs were influenced by the Norse knarr, which was the main trade vessel in northern Europe at the time, which used a steering oar. Cogs could carry more cargo than knarrs; the rudder made steering easier than did the steering oar of the knarr, especially for larger vessels; and cogs were cheaper to build. The latter was due to the cog's use of sawn, rather than split, timber. Fore and stern castles would be added for defense against pirates, or to enable the use of these vessels as warships. The stern castle also afforded more cargo space below by keeping the crew and tiller up, out of the way.