It’s a National Historic Site (since 1925) and a UNESCO World Heritage Site (since 2007).
It’s the oldest continuously operated canal system in North America, apparently.
It offers the world’s largest naturally frozen skating rink, assuming just the right mix of temperatures and the expenditure of a lot of money for flooding and snow clearing.
It was built to provide “a secure supply and communication route between Montreal and the British base in Kingston,” bypassing the bit of the St. Lawrence bordering New York state, in case of an American invasion. Opening in 1832, it also offered an alternative route for commercial shipping from Montreal to the Great Lakes.
These days, it serves as a 202-km-long navigable system for pleasure boats.
It was built using natural, navigable bodies of water to the extent possible–the Rideau and Cataraqui Rivers and several lakes–using locks where necessary.
Most of the men who died in its construction died from malaria (!), rather than what we would call workplace accidents.