It's hard to imagine that Skibo Castle, already a favorite old pile of ours in the United Kingdom, could find room for improvement. Now the hideaway in the Scottish Highlands—a private club that admits guests for one-time visits only and was once the former summer home of Andrew Carnegie—has added a spa. The $2.4 million project is, in fact, just part of a larger makeover (room redecoration, a restaurant rethink) of the 7,500-acre estate north of Inverness. "If you wish to be a world-class destination," says managing director Peter Crome, "you can't rest on your laurels." For the spa, which occupies a converted Edwardian garage where Carnegie kept his Bentleys, Skibo's designers have smartly chosen to depart from the chintz and heavy draperies of the 21-room castle. Instead, calming modern luxe is the prevailing style, with a slate fireplace and a Highlandesque palette of pastel blue, green, and heather purple. The three silk-lined treatment rooms include marble showers, heated floors, and one-of-a-kind handblown-glass sinks. The offerings—from antiaging facials to full-body mud therapy—don't break new ground, but they're expertly administered using products from the Italian maker Comfort Zone and Sodashi from Australia. We're not sure which we found more soothing: the 80 minute hot-stone massage or the sight of swans gliding by on Lake Ospisdale during our manicure. If you can't relax here, you can't relax. From $1,175 to $1,730; treatments, $15-$145. (Nonmembers can stay once but must join for future visits.) In Dornoch, Sutherland; 44-1862/894-600; www.carnegieclub.co.uk.