Dundurn Castle is located on lands formerly owned and occupied by Richard Beasley (1761-1842), merchant and politician, who settled here in 1793. Although the Castle incorporates the former Beasley farmhouse, it is now closely associated with its builder Sir Allan Napier MacNab (1798-1862) – soldier, lawyer, businessman and Prime Minister of the United Provinces of Canada (1854-1856) – who named it after his Scottish ancestral seat.

Built to the design of local architect Robert Wetherell, the residence features a combination of Classical and Italianate features that have been alternately described as both Regency or Italianate Villa. Constructed of brick and covered in stucco, the Castle is distinguished by French windows, broad verandas, two three-storey square towers, slate roofs and a later prominent Doric portico at the south entrance.

Set on Burlington Heights, remnants of a War of 1812 fortification are still evident and incorporated into the landscape designs of Robert Wetherell, William Reid (1833-1834) and George Laing (1854-1857). The landscaped grounds and outbuildings – including a dovecote, coach house, stables, cockpit and gardener's cottage – are considered an important example of the Picturesque Movement in Canada.

In 1977, the City of Hamilton designated the Castle under the Ontario Heritage Act and, in 1983, the Ontario Heritage Trust secured a heritage easement to conserve the building and grounds. Dundurn Castle was designated as a National Historic Site in 1984.