Begun in the 1380s, this remarkable ruin is one of the finest examples of an old keep enlarged with other buildings of a later date. The builder of the keep was Sir Simon Preston, whose coat of arms is above his original front door. It became a frequent residence of the Stewart kings and was often used by Mary Queen of Scots, being within easy reach of Holyrood. In 1543 it was capture by the English under the Earl of Hertford, who burnt it and demolished much of the stonework. The part used by Queen Mary was, therefore, but a remnant of a once much grander building. In the 17th century it passed into the hands of the Gilmours, since when some efforts have been made to prevent further deterioration of the fabric.

The castle of Craigmillar is one of the best examples of an old keep later extended into a courtyard and surrounded by an outer courtyard wall and garden enclosures. The interiors, such as remain, exhibit strong features of design and decoration. The Great Hall has a fine fireplace with an ashlar canopy supported on fluted pilasters. The Chapel, dating from the 15th century, is so placed as to be accessible both from the castle and from the outer courtyard.