This building is sometimes called The Castle on the Grand, though it's not really a castle at all-- it's the prettiest secondary school in town, and one of the oldest continuously operating ones in Ontario.
Galt Collegiate Institute was founded in 1852, with the main structure of its current building built in 1854. There have been large additions made to it since, and obviously the interior has been modernized a ton, but you can't really see the changes much from the street. The original (1854) part of the building was built in the Scottish Baronial style, on land donated by the founder of the (former) town of Galt. It backs onto the Grand River and the fossilized limestone used to make both the original building and some of its earlier additions was quarried on site from the riverbanks.
Since this school is so old, there have been lots of stories over the years about it being haunted-- I even came across it in a book of ghost stories collected from around North America once! The school seems to have embraced the lore, with its teams being known as the Galt Ghosts. Its current student population is about 1000 kids from grades 9 to 12 (born between 2005 and 2009).
