Those heading south on King Street during the late colonial years wouldn’t have seen fancy restaurants and hotels as they approached the city.

Instead, they would have been met with a tabby fortification about the size of Fort Sumter.

This fort, which was called the city’s Horn Work, is long gone, with only one above-ground remnant visible today, on the western side of Marion Square.

Historian Nic Butler said that 6-foot-tall by 10-foot-wide slab is the only part of the city’s colonial fortifications that people don’t have to pay admission to see, and it gives a rather incomplete picture.

“Tourists walk up and look at it and shrug their shoulders and walk away. Locals do the same thing,” he said.

Butler’s recent research gives a fresh look at what once was a very substantial work, an enclosed fort that covered 5 acres, harbored 18 cannons, and was surrounded on the north side by a moat 10 yards wide.

The Horn Work’s name stems from its two protruding points that faced north and surrounded those hoping to pass south on King — the original road into and out of the city. While its symmetrical design appears to be a random set of angles, Butler said it was carefully based on geometry.

“The whole idea of this thing is to protect the road into town,” he said. “Yes, it was a restriction to civilian life, but it was built with military needs in mind.”

Butler has done fresh 3-D modeling and tracked down fresh maps from archives in California and England — information he plans to share during a lecture at 6 p.m. Wednesday at the Charleston County Library.

The Horn Work also has similarities to Fort Littleton, a tabby fort on Port Royal outside Beaufort. Both were designed by Emmanuel Hesse. In both cases, tabby walls were constructed in front and back, with about 16 feet of interior space filled with dirt — taken from an area that would become a moat.

Ultimately, the Horn Work wasn’t enough to keep British forces from prevailing during the siege of Charleston in 1780.

The fort, which had stood since 1759, was taken down in 1784, shortly after the Revolutionary War.

Some of the tabby fortification survives underground, according to earlier archaeological digs.

However, experts continue to try to get a more complete picture on just how much remains buried. Two years ago, College of Charleston geology professor Scott Harris worked with a team using ground-penetrating radar to try to get a more complete picture.

Katherine Pemberton of the Historic Charleston Foundation said Butler’s research on the Horn Work and other 18th-century defenses will serve as a good foundation for a book on the city’s earliest fortifications, “which we desperately need.”

“You don’t think of downtown Charleston being a battlefield, but it was,” she said.

Butler said raising awareness about the Horn Work could help get archaeologists involved whenever there’s a need to dig or disrupt some of the area where the fortification once stood.

“I want to give people an idea of how massive this thing was so they can get excited about it and protect it,” he said.

Reach Robert Behre at 937-5771.