He was also ill. Before his stay at the Fairmont, he’d spent two months in Merritt Hospital in Oakland for appendicitis and a prostate-kidney infection. He received the Nobel Prize in his hospital bed. More concerning was the longstanding tremor in his hands, which was starting to make it difficult to steady a pen. This isolated house, with Carlotta acting as secretary, nurse and guard, was intended to free O’Neill to write.

“This is final home and harbor for me,” he wrote a friend. “I love California. Moreover, the climate is one I know I can work and keep healthy in.”

Tao House was built on a field overlooking rolling hills and Mount Diablo. The first building I saw was the Old Barn, which has been converted into a theater. Every August and September, the Eugene O’Neill Festival puts on plays in the barn and downtown Danville. This year, from Oct. 11 to 14, there will be a second festival in New Ross, Ireland, near where O’Neill’s father was born. As our guide, Victoria Ramirez, led us toward the house, a bevy of quail ran across the lawn, plume feathers bobbing.

Tao House is an odd mix of Spanish colonial architecture and Chinese details. The name comes from the Taoist concept of The Way and reflected Carlotta’s somewhat shallow interest in Asian philosophies. The brick walkway zigzags to thwart negative energy, which is said to flow in a straight line. The shutters and door are red, for good luck. The Spanish roof tiles are black to mimic Chinese architecture. Carlotta later said that she wanted to build a Chinese house but ran out of money, so “built a sort of pseudo-Chinese house.”

Tao House is cavernous, with white walls, dark blue ceilings and brown tile floors, to symbolize the sky and earth. Built-in bookshelves once held 8,000 books. Masks line the staircase, a nod to O’Neill’s play “The Great God Brown.” (The masks aren’t original, but were once owned by Andy Warhol.) In the music room, there’s a reproduction of O’Neill’s player piano, Rosie, a happy reminder of his barroom days. In his bedroom, there’s a Chinese opium bed, one of the few original pieces in the house, thanks to Katharine Hepburn, who convinced the furniture store Gump’s to donate it to the park. “Can it mean that much to you?” she wrote to the store. “Think and be kind — you are Gump’s!”

When I walked into O’Neill’s study, a woman on the tour sighed. “I could write in here,” she said. Before us were two big desks with a chair between them. Bookcases lined the walls. The wood paneling was reminiscent of the captain’s cabin on a ship, hearkening back to O’Neill’s carefree time as a sailor. A screened-in porch looked out at the view. The room was the epitome of peace and quiet. On the desk was a pack of prewar Lucky Strike cigarettes, thought to have belonged to O’Neill.

Every day, O’Neill struggled against the worsening tremor in his hands. He had a rare neurological disease, late-onset cerebellar cortical atrophy, that was misdiagnosed as Parkinson’s. The more he tried to write, the worse the tremors became — sometimes the pencil would fly out of his hand. O’Neill knew the disease was genetic, calling it “a heritage of God knows how long a line of people with high-strung nerves.” Ms. Ramirez showed us how his handwriting changed over time. As the disease progressed, his script became so cramped it could only be read with a magnifying glass.