THE TOWER and star-piercing turrets of Lyndhurst Castle float on a knoll high above the Hudson River. The gray-white marble castle could be the one from which Lochinvar rode west or the tower from which the maiden let down her long hair. Lyndhurst is the castle drawn from everyone's imagination.

Its third owner was Jay Gould, the tiny tyrant of money machinations in the last quarter of the 19th century. Lyndhurst was built in 1838 for Gen. William Paulding, once mayor of New York City, to a design by architect Alexander Jackson Davis. It is considered the finest and earliest American example of the Gothick Revival taste. The "pointed style," high fashion of the day, was imported from England where it was more a literary invention of Horace Walpole and Sir Walter Scott than an architectural revival.

In 1864, Davies designed an addition, a tower and banquet hall, for merchant prince George Merritt who had just bought the place. The completed structure was notable not only for its exquisite and delicate Gothic forms but also for its furniture, most of it designed by Davis for specific places in the castle.Much of the original furniture has been lost, but subsequent owners brought other pieces. As a result, Lyndhurst is a museum of successive furniture fashions of the late 1800s: Gothic Revival, Victorian Gothic and Beaux Arts.

Gould, though wealthy, was not acceptable to the older money of the Vanderbilts and the Astors. He might have bought Lyndhurst in 1880 as his country place with a view toward living in the manner befitting his millions. Though he did not entertain lavishly, he lived in great comfort and style there and in his more modest brownstone in New York City, until his death in 1892. His daughters preserved the house and much of its land and left it to the National Trust for Historic Preservation. The Trust takes care of the estate, welcomes all sorts of events from dog shows to historic houses workshops on its grounds and holds it open to the public.

Gould is the subject of a small and pleasant exhibit at the National Portrait Gallery that continues through Feb. 4. Fred Voss and Michael Lawson, Portrait Gallery research historians, put the show together. They looked at the pictures of Lyndhurst, collected paintings of Gould's friends and enemies and sorted through the newspaper cartoons of the day that generally lambasted him as a robber baron. They wound up with quite an affection for him.

The newspaper and magazine cartoons in the exhibit deal with aspects, some of them dubious, of Gould's business life. He tried to corner the gold supply. He gained control of Western Union. He put together a vast network of railroads. He lost and gained several fortunes in stock manipulations and left about $80 million in the end.

The photographs of the period are perhaps the best part of the show, though there aren't enough. The three or four photographs of Lyndhursts are, for castle buffs, gilt-edged glimpses of the Gilded Age. The 40-room mansion stood on a 550-acre estate near Tarry-town. There are outbuildings to hold a bowling alley, indoor pool and carriages.

The greenhouse-in the Turkish style with its great onion dome-is shown in an interior view. There 15 gardeners tended what was said to be the largest collection of orchids in the world, as well as 2,000 azaleas. Nowadays, the Trust keeps a hand extended for money to reglaze it.

Another photograph shows what is the glory of the house - the second-floor, 40-foot-long art gallery and sometime billiards parlor. The paintings, most of them in the French and German academic styles, were mostly of picturesque peasants, serene sheep and high-spirited horses. They are hung in the manner of the day, two and three above each other. Today, the paintings, which cost quite a lot at the time, are not universally admired. As Voss and Lawson put it on the label, "In Gould's case . . . daring in business did not carry over into art collecting."

The room is lighted by a tall window set in a Gothic arch and by a stained-glass skylight. Side windows as well are of stained glass. The ceiling is vaulted with heavy ribs reating on elaborately carved corbels (brackets). Heads of Washington, Franklin and Shakespeare ornament the corbels.

Gould's yacht, called Atalanta as was his railway car, is a fine ship. He used it not only for pleasure, but, thrifty man that he was, for commuting to New York. A watercolor from Lyndhurst and photographs of the period stateroom is palatial. The dark furniture is much like that of Lyndhurst. Mirrored wardrobes are embellished with brass and carvings. The bunk is actually a cabinet bed, heavily draped with a padded wall and a fat, paddled stool to step up on. The ceiling is a marvel with all sorts of beams and coffers. Of course there is an Oriental rug on the floor.

Five of Jay Gould's six children are shown in an 1877 picture. One daughter, Anna Gould (who was often pictured with a huge ostrich-tail hat and a fur muff big enough to climb inside), married not one but two titled Europeans, ending up as the Duchess of Talleyrand-Perigord. Her first husband was notable for spending her $6-million dowry in the two years they were married. It was the duchess who gave the castle to the Trust.

An elaborate table, just inside the door of the Portrait Gallery exhibit room, was commissioned by Gould for the parlor. The label speaks of it as being in the Greco-Roman style, but that hardly says enough. The tabletop is elaborately inlaid with marquetry. In the center are flying birds. Baskets of flowers adorn the north, east and west ends. Blossoms bloom on a border. And the whole affair sparkles with brass stars and scrolls. The base has knobby legs, like someone's fat uncle. And, sad to say, there's a bit of curlicue lost to posterity on the side judiciously placed away from the door.

A settee commissioned by Gould for Lyndhurst is also on exhibit. Some people have tried to say that the settee is scaled low to the ground because Jay Gould was likewise - he was but 5 feet tall. The truth is, a great deal of furniture of that period had short legs and shallow seats. The settee, if possible, is even more marvelous (hilarious might be the better word) than the table. On its knees are carved faces of fat Egyptians. In the middle of the back is a medallion of a classical blond, suitably framed in curlicues. The seat is covered with a chinoiserie red brocade. There is a matching chair, to confound those who might understandably believe it couldn't be done twice.

Dominating the room is a quiet, thoughtful portrait of Gould, painted in 1896 by Eastman Johnson. The portriat is on long-term loan to the gallery from the New York University Art Collection. In return for the loan, the Portrait Gallery restored the painting.

This charming show, tucked away in a single, pink-painted room, is a most pleasant way to learn a bit about a colorful period of history. But it is that picture of Lyndhurst, in all its Gothic grandeur, that draws you back again and again. It might have been worth being Gould to have owned Lyndhurst - even though he spent many a sleepless night pacing up and down in front os his house, bodyguard at the ready. CAPTION: Picture 1 through 4, Jay Gould's Lyndhurst Castle holds court on a knoll high above the Hudson. Gould, filled it with furniture of every kind and pieces like the table, and settee, being cleaned by Michael Lawson and Fred Voss, can be seen at the National Portrait Gallery. The Gothic lines of the castle are evident in its picture gallery, left,

Table and chart photos by Margaret Thomas-The Washington Post;

Picture 5, The master bedroom is typical of the Gothic splindors at Lyndhurst. Gould's jewelry box, above, is a model of his private railroad car., Above photo by Margaret Thomas - The Washington Post