Bob Cassilly sprinkled the nation with eccentric sculptures and breathed new life into downtown St. Louis by opening a museum where more than a half-million tourists go every year to see such oddities as 4-foot-wide Slinkys and a 76-foot-long No. 2 pencil.

Mr. Cassilly, who was 61, was found dead Sept. 26 on the site of his latest project, an amusement park to be called Cementland. The St. Louis Post-Dispatch reported that he had suffered serious head wounds after the bulldozer he was driving apparently toppled over before landing upright.

Cementland was perhaps the most extravagant of his innovations — a phantasmagoric world where, in carefree fashion, visitors could throw rocks from atop a smokestack more than 200 feet high and ride in kayaks through man-made waterways in what used to be an industrial wasteland. Obsolete heavy machinery would churn away just for their viewing pleasure.

Mr. Cassilly had planned it all — plus a collection of his signature animal sculptures — for a lot on the St. Louis riverfront once occupied by a cement company.

A classically trained sculptor, he started small, relatively speaking, with outsize figures inspired by the animal kingdom. Over his four-decade career, he populated his native St. Louis with creatures including serpents fit for a fantasy novel. For a Dallas restaurant he built a hulking gorilla. New York City got hippopotamuses.

His most famous creation, the City Museum that opened in St. Louis in 1997, pulled his creative energy, or at least most of it, under one roof. It houses such wonders as a “ monumental, monolithic, monstrous montage of monkey bars ” and “a collection of vintage (working) shoelace machines.” On top of the roof sits a gigantic praying mantis (a Cassilly original) and a Ferris wheel.

The New York-based nonprofit group Project for Public Spaces in 2005 named the complex one of the great public spaces in the world. It rates apage in the travel guide “1,000 Places to See in the United States and Canada Before You Die.” And it cured the boredom of untold numbers of children, and their parents, during long family road trips.

Mr. Cassilly and his then-wife began work on the museum at a time when others were raking in profits through demolition in the city center, where crime and unemployment had reached crisis levels. The couple acquired an old shoe factory and, with remnants of old St. Louis structures, made the City Museum rise phoenixlike from the ground.

Mr. Cassilly said he was undeterred by doubts that his creation would draw people to an area then perceived as dangerous.

“We’ll get the people who aren’t afraid” to visit, he told the Wall Street Journal in 1997.

Today, general admission is $12, not including extra fees to visit the roof and the 13,500-square-foot aquarium. In keeping with the cheeky spirit, he once put up a sign steering customers to pony up at “Greedy Bob’s Parking Lot.”

Mr. Cassilly often worked without permits or blueprints, and the City Museum was sued more than two dozen times for injuries, some of them serious ones suffered by children, that occurred on the premises.

Mr. Cassilly defended the museum— a place where all visitors, for the brief time they spent there, could have fun generally reserved for kids.

“Statistically, we probably have fewer injuries than a playground,” he told the Post-Dispatch in 2007. “If you are going to do anything in life, there is a risk.”

Robert James Cassilly Jr. was born Nov. 9, 1949. His father was a building contractor and gave him access to a full set of tools when other boys his age were playing with erector sets.

Mr. Cassilly spent his childhood playing near what he called the “creek” — a drainage area behind his family’s house — where he built caves and tunnels. He even persuaded his grandmother to snake her way through the underground world, his mother, Judith Cassilly, said.

Mr. Cassilly excelled in the Boy Scouts and earned his first merit badge in wood-carving; he whittled neckerchief slides for Scout uniforms.

When he was 14, he got a job sweeping floors for a St. Louis professor and sculptor, Rudolph Torrini. Seeing the boy’s potential, Torrini gave him a chisel and a mallet and taught him to carve like a professional.

Mr. Cassilly received bachelor’s and master’s degrees in fine arts from what is now Fontbonne University in St. Louis.

His marriages to Cecilia Davidson and Gail Soliwoda ended in divorce. Besides his mother, survivors include his wife of seven years, Melissa Giovanna Zompa Cassilly of St. Louis; two children from his second marriage, Daisy Cassilly and Max Cassilly, and two children from his third marriage, Dylan Cassilly and Robert Cassilly III, all of St. Louis; two brothers; and a sister.

In 2007, he told the Post-Dispatch that, with his attention turned on Cementland, he didn’t know how long City Museum would go on.

“You shouldn’t assume things are going to last forever,” he said. “It would be great if it all collapsed onto itself like Camelot. We would have had this brief shining ‘ah’ moment. But that’s just the romantic in me.”