SINGAPORE - Since he first stepped into Fort Serapong in 2011, exploring enthusiast Helmie Khalid has been back to the wartime fort five more times.

"I could go back countless times, because we've discovered only 20 per cent of the whole fort," said the 29-year-old civil servant.

Built in the 1870s, Fort Serapong, together with Fort Siloso, Fort Connaught and Imbiah Battery, formed the British army's southern coastal defence on Sentosa. An artillery battery is a platform from which guns are fired.

When World War II broke out in Singapore in 1942, Fort Serapong and all other coastal batteries stood unused, as a seaborne attack from the south never came.

Mr Helmie is part of local exploring group T.R.E.E. or Temasek Rural Exploring Enthusiasts, which started in 2014.

From heritage trails to nature treks, the 11-member group leads guided tours of about 30 people each time.

In this episode of Living City, which explores Singapore's overlooked spaces and places, T.R.E.E. takes Straits Times Video on a journey into Fort Serapong.

"Fort Serapong is almost untouched. You can see the wear and tear on the fort, and the mechanisms that used to work," said Mr Helmie.

Despite his many visits there, he is not tired of the place. "It never gets boring. It's one of those places where you seem to discover a little bit more every time you step into it."

But the urban explorer warned that it would be risky for people to enter the fort alone, as the structure is unstable.

People who want to explore the fort should contact T.R.E.E (www.temasekexplorers.com) for guided tours.

"If you were to go alone, accidents might happen. Since we've been here so many times, we can help ensure safety and offer historical facts, so explorers can fully enjoy this fort."

Correction note: In an earlier version of the story, it was stated that T.R.E.E works closely with the Sentosa Development Corporation. Sentosa clarified that they do not work closely with the explorer group T.R.E.E. We are sorry for the error.