Since he first stepped into the ruins of Fort Serapong in 2011, Mr Helmie Khalid has been back to the wartime fortress five more times.

"I could go back countless times, because we've discovered only 20 per cent of the whole fortress," 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 sea attack from the south never came.

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

From heritage trails to nature treks, the 11-member group leads guided trips involving about 30 other enthusiasts each time.

In this episode of Living City, T.R.E.E takes Straits Times Video on a journey into Fort Serapong.

The 10-part video series explores overlooked spaces in Singapore.

"Fort Serapong is almost untouched," said Mr Helmie.

"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 intrepid explorer warned that it would be risky for people to enter the abandoned fort alone, as there are collapsed concrete structures and rusting metal pipes.

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

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.