Humewood Castle was originally conceived as 'an occasional resort in the summer recess or shooting season. Albert Kimberley, a builder/contractor, submitted a 13,650 bid based on White's first sketches of Humewood. White continued to enlarge and improve upon his original designs. Without interfering, Mr. Dick watched his castle grow believing that all would be covered by the original estimate. In 1870, Mr. Dick received the Kimberley's bill for 25,000 and refused to pay the difference. Kimberley sued both White and Dick, and won.

Humewood is a Victorian hybrid: a Scottish baronial hall with Irish battlement detail. Its asymmetry and spectacular mass of local granite is punctuated by battlements and pointed towers. The castle is built of granite, a material that White knew and liked. "In the treatment of granite especial care is required to make the moldings of a broad, bold and massive, rather than a small or delicately undercut character and to avoid as far as possible anything like minuteness and pettiness to the finish," a dictum followed by White at Humewood. It is a masterful composition of triangular and pyramidal forms, building up to a 100-foot turret rising above the corner of the rectangular central tower.

An impressive skyline is a characteristic of Victorian architecture as evidenced by Humewood's silhouette of horizontals and curves, crenellated towers, spirelets and gables which was dramatically set against the backdrop of the Wicklow mountains. White, paying attention to the danger of attack by the Fenians, built a basement with strongly barred windows, that also served to keep the damp from the main part of the house and a "porte-cochere" where guards in the room above it could command the entrance through firing holes pierced in the vaulted roof of the porch.