He was, a Harvard chum once said, "a man of unusual character and imagination, handsome, winning, interesting -- and odd."

That Henry Chapman Mercer was odd -- or "plain crazy," as some people thought -- is one of the enduring myths of Bucks County, Pennsylvania, where he spent his life. What else could account for the Mercer Mile, three buildings he left behind in Doylestown?

One is his museum, a concrete Gothic hulk in the center of town that holds a maelstrom of rusting tools, the world's largest collection of 18th and 19th Century American implements and artifacts. Another, about a mile away, is his tile works, also wrought of concrete, a sprawling mix of medieval and mission that harbors kilns and clay. The third, visible from the tile works, is his house.

It's the house, an extravagant castle in concrete, built over a commonplace 18th Century clapboard, that caused the greatest uproar in pastoral Pennsylvania. From 1908 to 1910, when it was under construction, the ladies' tea in Doylestown could speak of little else. It cemented Mercer's fame as an eccentric.

Walking inside this creation, which stands in isolation on the outskirts of town, about three hours' drive from Washington, is like stepping into Henry Mercer's brain: columns scattered helter skelter to sneer at symmetry; 3,000-year-old Etruscan pots, suspended in turkey wire nets from the ceiling; chests of drawers imbedded in concrete protusions. "Wild" is the word that occurs to most visitors as they climb careening staircases and marvel at the patented Mercer tiles that blanket every surface. Wild and fantastic.

"A lot of vaults I'm throwing together to call a house in reinforced concrete on a hill -- Mercer's Folly quite the thing," he wrote to his good friend, Sir Charles Hercules Read of the British Museum, in 1909. "Ceilings, floors, roofs, everything concrete. You stand up a lot of posts -- throw rails across them -- then grass -- then heaps of sand -- shaped with groined vaults then lay on a lot of tiles upside down & throw on concrete. When that hardens pull away the props & you think you're in the Borgia room at the Vatican."

The Moravian Pottery and Tile Works and the Mercer Museum, both built by the same novel method, would come later, but by 1912 when Mercer moved in, his castle was already drawing gawkers from far and wide. There was talk among the townspeople of secret chambers and passageways; there were inevitable, dark rumors of strange doings. Even today, half a century after Henry Mercer expired at 73 in one of his tile-encrusted bedrooms, the thing provokes gasps.

He dubbed it Fonthill, after the Virginia homestead of distant relative, though his notebooks propose other names -- Brevose Stanhope, Batshill, Appledore, Ellora, Mogadore -- that might have suited as well. By the seat of his Saville Row pants (because the archictecturally unscholled Mercer disdained blueprints), he built it for $32,916, and set about filling it up with his enthusiasms and obsessions. Made for Mercer and no other, it boasts such features as 10 bath rooms, innumerable concrete bookcases and a huge wine cellar, but lacks a dining room.

He was born in 1856 in Doylestown, the scion of the very proper Chapmans and Mercers -- judges, politicians, diplomats, social lions -- families that had prospered in Bucks County since the late 17th Century. Mercer never married. His younger brother, Willie, a country squire who dabbled in sculpture, produced twin boys who died in infancy. His younger sister, Lela, became an Austrian baroness. By World War II, the great Mercer/Chapman line had been virtually wiped from the map.

"I think this closest living relative is his grand nephew, a certain Baron von Friesen who lives in England," says Jane Acton of the Bucks County Historical Society, of which Mercer was president. "Most everyone who knew him personally is either senile or dead."

But the memories still are vivid. There was Mercer the boy, who liked to predide over children's tea parties at the house of his maternal grandfather, a judge and one-term congressman named Henry Chapman, and show his young friends the old judge's lithographs -- "romantic pictures of the Rhine Castles," as Mercer later recalled.

There was Mercer the young man, on an adventure with a Harvard '79 classmate through Europe and Egypt, both of them ending up at a whorehouse in Alexandria. "We found ourselves among the dancing girls -- dressed in their blue and white gowns, adorned with blackened eyebrows and lashes, brass trinkets and henna," reads the 1881 journal entry, exhumed by Fonthill assistant curator Linda Dyke. "Rattling their tambourines they began to dance while an old woman seated on the ottoman sang a monotonous song keeping time with a tomtom. As they danced neither very gracefully, nor originally, their garments one by one" -- and there the entry ends.

In later years, convinced that he'd be famous ("if I succeed in building the castle, I shall be remembered" is the gist of a French inscription, one of many that grace Fonthill), Mercer took scissors and pen to his private papers, cutting out or marking over the juicier parts. "He was really the very model of a Victorian gentleman," says Helen Hartman Gimmel, a Bucks County matron who's become a Mercer fan. "He wanted to be remembered, but only selectively."

As he grew older, Mercer went wherever his passions dragged him; being the collector that he was, he hung on to every one. Without the benefit of formal training, Mercer juggled the professions of archaeologist, anthropologist, architect, artist, master potter, naturalist, historian and even fiction writer. Though the latter came to naught, Mercer liked to think of himself as a great American writer in the tradition of Poe and Bierce. He was never strong on modesty.

In his November Night Tales, a 1928 anthology of short stories -- "spooky," Jane Action offers -- the narrator is a thinly disquised Mercer. "I saw the ladder move, and with a quick premonition of danger, stepped into the high empty cupboard," he recounts in "The North Ferry Bridge." "As I did so my legs grazed some bars of iron, and instinctively I stopped down and seized what I found to be a pair of blacksmith's tongs." ("When I read that," says Lynne Poirier, director of the historical society as well as the museum, which has more than its share of blacksmith's tongs, "I just roared and roared.")

Mercer was cool to children and most adults, but loved dogs other animals (a large furry spider and a bat had the run of his castle). He refused to drive a car, but often bicycled, his Tyrolean cape billowing behind him as he pedaled. He hated electric lights and the stylized pruning of trees, but loved weeds -- so much so, says Helen Gimmel, that when a lady fromthe Doylestown Nature Club impulsively plucked a weed from a shrub at Fonthill, Mercer waited till she'd gone, then carefully restored it. He was obsessed with fire, hating it (he chose concrete for his buildings because it was fire-resistant) as well as loving it ("the gift of fire" was a recurrent theme in his tile designs). He was obsessed with the past, and feared that the past was slipping away. His favorite newspaper, often the only one he read, was The London Times.

He was a proud man, headstrong, pedantic, and given to delivering lectures. His temper was sharp but -- as a younger acquaintance, Joseph E. Sandford, remembered in a monograph -- "not noisy." He kept shy of politics and hated the intrigues of academe. He was cheap in small things -- once offering to buy an employe a beer, then making the poor fellow pay -- but generous in large pursuits, finacing the museum and several ambitious archaelogical expeditions from his own pocket, letting his housekeeper and her husband live in the castle for perpetuity, and willing them the tile works to do with as they wished.

The ladies he admired from afar. In his youth, when the mustachioed Mercer was counted among the eligible bachelors of Philadelphia society, he loved to dance and party -- "shake a gay leg," as folks said then -- but he ended his days as a recluse.

Most of the above -- and there's much more to Henry Mercer -- is only hinted at in the museum displays, do-it-yourself affairs with scant labeling on six musty floors, while it's merely mentioned at the tile works.

The museum's permanent collection, called "The Tools of the Nation-Maker," has some 40,000 pieces, the bulk of which Mercer collected in Bucks County, some of which was added after he died. Among these items, some of them hanging upside down from the ceilings, are straw baskets, chairs, decorative iron stoveplates from the Pennsylvannia Dutch, a whaleboat lashed to a railing about 30 feet off the floor, apple pearers, medical implements, rakes, hoes and, in macabre twist, a gallows astride a restored 18th Century schoolroom.

It's dark as a dungeon, an improable but irrestible clutter recalling the warehouse of Charles Foster Kane. There's even a sled, though not "Rosebud." And everything is numbered, sometimes in Mercer's hand.

The museum shop sells a variety of Mercer books, such as his illustrated study of stoveplates The Bible in Iron or his treaties on Ancient Carpenter's Tools and The Dating of Old Houses, as well as individual tiles produced from Mercer's original molds.

Every spring, the Bucks County Historical Society holds the Mercer Museum Folk Fest, a celebration of wood turning, soap making, candle dipping and other vanished skills on the museum's front lawn. This year's, with a $4 admission charge, will be held May 9 and 10, 10-5.

The Moravian Pottery and Tile Works, owned and operated by Bucks County and still making tiles, presents a step-by-step explanation of the Mercer process by way of a slide-show and lecture, plus a self-guided tour through the catacombed building. It was here that Mercer conceived the tiles that still grace the Pennsylvania Capitol, the casino at Monte Carlo, the Pocantico estate of the Rockfellers and, of course, Fonthill. Of some 2,000 Mercer designs, drawing from sources as ecclectic as Indian lore and medieval heraldry, about 700 are currently in use. A catalogue, with prices ranging from less than $5 for a single tile to more than $400 for a large mosaic, is available at the office.

The castle, though, is where you'll meet Henry Mercer the men. His presence is overwhelming, and the feeling can be heightened by a knowledgeable docent. "Every time I show people around," says Jane Acton, "it's as if Dr. Mercer is looking right over my shoulder."

But the best tour guide, doubtless, was Laura Swain, Mercer's loyal housekeeper, who thought of him as something of a god. She came to him as a 16-year-old farm girl, married the foreman of his tile works, and presided at Fonthill like a queen until she died just six years ago.

"She used to answer the door wearing ankle socks and gym shoes," says Acton. "If she liked you, she'd let you take your time, but if she didn't, she'd have you out of there very briskly in eight minutes. She used to prod people along with a walking stick and say, 'It's time for the next room now."

Helen Gimmel recalls, "Laura had a few strange habits. For instance, She would wear white gloves for dusting, and rub the furniture, window sills, whatever, very vigorously with her gloved hands. She was actually very nice, though -- and odd."