An abandoned chalk pit, which has been an impenetrably dense jungle of bramble, gorse and fallen trees for more than a century will this month be revealed as an extraordinary lost garden created by two former prime ministers.

The lost garden of Walmer Castle in Kent was laid out in the early 19th century by William Pitt the Younger and completed by Lord Liverpool.

It was a magical place for peace and quiet, a spot perhaps to walk while having important discussions about matters of state. But for at least 100 years it has been overgrown.

Facebook Twitter Pinterest Walmer Castle, built during the reign of Henry VIII. Photograph: David Levene/The Guardian

“You couldn’t get in there,” said Mark Brent, English Heritage’s head gardener at the castle. “It was literally scrambling through brambles that were head high and fallen trees … you couldn’t have worked your way through it. It would have defeated an army.”

Brent, together with senior properties historian Paul Pattison, have led a project to bring the garden, known as the Glen, back to life and back to how Pitt imagined it.

Between 1792 and 1806, Pitt lived at Walmer Castle, the official residence of the Lord Warden of the Cinque Ports, a ceremonial post occupied by figures that have also included the Duke of Wellington, Winston Churchill and the Queen Mother.

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In 1802, along with his niece, Lady Hester Stanhope, Pitt set about creating new pleasure grounds on the exposed chalky land, battered by winds and salt spray from the Channel.

Facebook Twitter Pinterest The former chalk quarry could previously only be glimpsed through a dense and tangled tree canopy. Photograph: David Levene/The Guardian

At the time, Britain was at war with France and there was a great panic about the prospect of a French invasion. Pitt was a leading figure in raising a volunteer army. They were worrying times, said Pattison, but creating new gardens was probably a welcome distraction for Pitt, a “stiff upper lip to carry on as normal”.

Deciding to create a garden in the chalk quarry, a big hole in the ground, was particularly ambitious. “This is pure chalk – the actual soil in here is pretty scant,” said Brent.

Letters from 1805 have been found which talk of plans to plant “‘fir trees, broom, creepers and evergreens’ - probably all that will grow there,” said Brent.

Facebook Twitter Pinterest View of Walmer Castle showing the cloud hedging. Photograph: David Levene/The Guardian

The garden scheme was started by Pitt, who died in 1806, and completed by his Lord Warden of the Cinque Ports successor, Lord Liverpool, who became prime minister in 1812.

The castle itself was built as an artillery fort in 1539-40, during the reign of Henry VIII, and became a perk of being Lord Warden of the Cinque Ports in the early 18th century.

That has meant different people in history coming to the castle and grounds and making their own mark. “This is a unique place,” said Pattison. “It is not like any other country house, it hasn’t had a family living here handing it down from generation to generation. This was in the gift of the crown.”

No garden plans from Pitt’s time exist but a plan from 1859 has been found and researchers are confident that it reflects how it would have looked.

Brent and his team have spent about a year clearing the quarry and he took the Guardian on an early tour ahead of its formal opening. “Normally a hole in the ground like this anywhere in England is filled with old sofas and cars but fortunately we didn’t find anything like that: the surprise was that it was relatively clean.

“It is a magical space … The survivability of some of the trees is amazing, some of the yew trees are ones which Pitt and Stanhope planted.”

Facebook Twitter Pinterest The lost garden was laid out by William Pitt the Younger and Lady Stanhope, and finished by Lord Liverpool. Photograph: David Levene/The Guardian

The opening of the glen is part of a wider £2.3m project to revive and reunite gardens begun by Pitt and Stanhope. It includes a £1.35m grant from the National Lottery Heritage Fund.

When the garden opens to the public towards the end of April, visitors will walk down a new metal staircase on to paths in the glen which follow the lines of the original. Bulbs which can grow in calcareous soil have been planted as well as broom, gorse and laurel.

“It would have been quite a secluded, contemplative place,” said Pattison. “I like to think we are doing exactly the same thing that Pitt and Stanhope were doing.”