Henry Pellatt purchased 25 lots on Davenport Hill in 1903, then commissioned E. J. Lennox -- the architect behind Old City Hall and other Toronto landmarks -- to design something unprecedented. Construction began in 1911 with the massive stables and the Hunting Lodge, a two-storey coach house where Pellatt lived while 299 workers spent three years raising the mansion itself. The castle sprawled across 64,700 square feet, incorporating an elevator, vertical passages for pipe organs, a central vacuum system, and hand-carved oak paneling in the style of Grinling Gibbons that took three artisans three years to complete. Italian craftsmen fashioned the ornate plaster ceilings. Five acres of gardens surrounded the estate. Then World War I broke out, and construction halted, leaving the third floor unfinished forever.

Pellatt's downfall came swiftly. During the postwar depression, the City of Toronto raised Casa Loma's property taxes from $600 per year to $1,000 per month. Already strained, Pellatt auctioned off $1.5 million in art and $250,000 in furnishings before abandoning the mansion in 1923. The city seized it for unpaid taxes in 1924, and for years the building sat empty, its grand halls echoing with nothing. Briefly, in the late 1920s, investors ran it as a luxury hotel. During Prohibition, wealthy Americans crossed the border to drink and dance there. The Orange Blossoms, who later became the Glen Gray and the Casa Loma Orchestra, played the venue for eight months in 1927-1928 before launching into a North American tour as a major swing-era dance band.

Pellatt was more than a financier with extravagant taste. He joined the Queen's Own Rifles of Canada as a common rifleman and rose through the ranks to commanding officer, eventually earning a knighthood for his dedication to the regiment. He later served as honorary colonel and was promoted to major-general upon retirement. Today, the unfinished third floor of his castle houses the Queen's Own Rifles Regimental Museum, moved there from Calgary in 1970 to honor his legacy. The second floor holds a Girl Guide exhibit dating to 1973, tracing a connection to Lady Pellatt, who hosted 250 Girl Guides for tea and a tour of the turrets as early as 1913. Rallies at Casa Loma became an annual tradition, and Guides even skated on the house's curling rink in winter.

Casa Loma's Gothic turrets and grand interiors have made it one of Canada's most filmed locations. It doubled as Wayne Manor in the DC series Titans, stood in for sinister mansions in Guillermo del Toro's Crimson Peak and the thriller Ready or Not, and appeared in X-Men, Chicago, Scott Pilgrim vs. the World, and The Rocky Horror Picture Show: Let's Do the Time Warp Again, for which a movie theatre marquee was erected at the castle's front entrance. Rush shot the cover of their 1981 live album Exit...Stage Left nearby. In 2022, Future and Drake filmed the music video for Wait for U within its walls. The castle that bankrupted its builder now earns its keep as a backdrop for other people's stories.

The Kiwanis Club of Casa Loma managed the property for 74 years, from 1937 to 2011, opening it as a public museum. A 15-year, $33-million exterior restoration ran from 1997 to 2012. In 2014, Liberty Entertainment Group took over under a new long-term lease, investing $7.4 million in further upgrades and opening Blueblood Steakhouse in the castle's dining rooms in 2017. Casa Loma sits on Davenport Hill, which traces the ancient shoreline of Lake Iroquois, the predecessor to Lake Ontario. From its towers, visitors look down the escarpment and along Spadina Avenue into the heart of Toronto, surveying the same view that once belonged to a man who built himself a castle and couldn't afford to keep it.