Trebuchet designed by Rob Cagnetta will be tossing pumpkins long distances today.

Storm the castle!

Or maybe you could just watch people practice in a park. This afternoon the West Broadway Neighborhood Association in Providence conducts its Goblins and Gremlins Parade at the Dexter Training Ground Park. There, of course, will be a parade in the park. And before that, there will be food and games, activities and live entertainment.

This includes the Black Knight. It’s not a person, but a thing: a trebuchet.

Picture a pendulum. Add weight to one side and a sling shot to the other.

Let it hurl.

Pumpkins are the preference. And for those who wonder why, the answer’s history.

“People have been throwing things since they found out they could throw things,” says Rob Cagnetta, the director and designer of the Black Knight project, which involved six people working (mostly volunteering) 500 hours over the last three months.

A trebuchet is an ancient weapon, related to the catapult, that was popular in the Middle Ages. This particular trebuchet, which was recently constructed, can fling projectiles more than 200 feet and is making only its second public appearance.

“I need a place where I can throw,” Cagnetta says. “I live in downtown Wakefield. I suppose I could throw over the fire station.”

This is the third time Cagnetta has constructed a pumpkin-tossing machine. The first time was in 2001 as a favor to a friend, who happened to work for the South Kingstown Parks Department and was having a pumpkin launching event. But she lacked a launcher.

“I said I can help,” Cagnetta says.

Cagnetta is president of Heritage Restoration in Providence, a construction company. So he knows about building things, although he had never built a large-scale medieval military device.

“This was a fun opportunity to figure out how to launch things using skills of carpentry and mechanics and physics. I found there is a community of people out there who design things to throw pumpkins.”

Cagnetta designed a catapult, which could throw pumpkins 80 feet. He called it The Pumpkinator.

“It was based on garage springs. It was very crude and it only lasted two years until it started to fall apart.”

So in 2003, Cagnetta built another contraption, but this time not a catapult, but a trebuchet, which has not only the flinging leverage of catapult’s arm, but a sling shot at the end of that arm, and it’s all powered by a swinging counterweight.

With workers from his company, Cagnetta constructed a trebuchet based on a model he found online, which he treated as an adaptable recipe.

“Some measurements were multiplied by four, some by 10. There was a lot of trial and error. There are some basic issues of weight and counterweight, sling length and pin angle. There are five or six variables in a trebuchet that make it perform well or poorly.”

This trebuchet, which was never named, could throw pumpkins 240 feet. But three months ago was the last Cagnetta saw of the device. He and his business partner parted ways, and his former partner got the trebuchet.

“I always wanted to make another one because the first trebuchet was not an accurate rendition and I wanted to make another one anyway. There’s no better reason to make one than not to have one.”

Cagnetta enlisted volunteers from his business, “who are doing two timber-frame jobs right now so their skills are honed,” and people from Providence’s Steel Yard to provide the steel joinery for the timber.

“If I went anywhere else and said I wanted to build a trebuchet, I’d have to explain what it was for an hour. At the Steel Yard, they just said, ‘Absolutely.’ ”

Trebuchets came into being about 500 B.C. and were used until the 1500s, until the advent of gunfire (and canons). They were used to hurl heavy objects into the walls of a fortress, or over it.

Now, they’re used for entertainment, and attract lots of spectators.

“The appeal for people is just to see things smash. It kind of builds a community, even though it was designed to destroy communities.”

Kids can pull the trebuchet’s firing pin and watch pumpkins sail through the air at targets hundreds of feet away.

“It’s like a game, a big pinball machine. There is also a fascination. They are looking at this machine that doesn’t work off anything but physics — weight and counterweight. It’s really graceful.”

The Dexter Training Ground Park is located between Dexter and Parade streets off Westminster Street in Providence. The preparations for the parade begin at 3 p.m.; the parade is at 4:30 p.m. For more information, visit www.wnba.org. To see a video of the Black Knight in action, visit www.youtube.com/

watch?v=s5EgIZZ6ZLM.

brourke@projo.com