Moscow, Russia | Descending into the basement of the 1930s-era Kremlin administration building is an eerie experience: the corridors are still lined with brown Soviet tiles and walls bear remnants of secret communication wires. 

But turning off to the left or right, concrete foundations have been removed from the humid underground storage rooms, giving a glimpse at some of the best-preserved remains of early Moscow's bustling mediaeval quarters. 

Human bones jut out from the earthy sides of the dig, leftovers from an old necropolis for the gentry who once lived here. Beneath the drab flooring, archaeologists have marked the layered strata: 12th century, 13th century, 14th century.

Since 2014, when President Vladimir Putin ordered the dismantling of a massive Stalin-era building known as "The 14th Wing" inside the Kremlin walls, archaeologists have enjoyed unprecedented access to the premises, says Nikolai Makarov, director of Moscow's Institute of Archaeology.

"There are many mysteries in the Kremlin," he told AFP during a recent visit. 

As the "territory of power," it has been largely inaccessible for serious archaeological research.

"There have been full-scale archaeological works here only twice," including currently, he says. 

- 'Ideological' destruction -Most of the Kremlin was closed off soon after the Bolsheviks moved their government there in 1918 from the tsarist capital of Petrograd, now Saint Petersburg. 

Today, visitors have to be scanned by the presidential security service before entering the 16th-century cathedrals inside the former fortress and UNESCO World Heritage site that houses Putin's office.

The 14th Wing became one of the first Communist footprints on the premises. It was built in 1932 after the Bolsheviks destroyed two of Russia's most revered religious sites, the Chudov monastery and the Ascension convent. 

Children of the royal family were baptised in Chudov monastery and the 17th-century reformist Patriarch Nikon was put on trial there. Royals, meanwhile, were buried in the necropolis of Ascension Convent while coronations took place in the square outside.

"This demolition was not just functional but ideological," said architecture historian Yulia Ratomskaya who specialises in medieval Russian architecture. The move was part of a campaign by the militantly atheist Soviet government to raze and repurpose churches.

- 'Frescoes buried in rubble' -The new Soviet government initially fixed up the buildings after they were damaged by artillery during the 1917 Revolution. But it then decided to replace them with a Red Army cadet school.

In a tragedy for Moscow's architecture, the 14th-century cathedral in the Chudov monastery was blown up in 1929 without honouring an agreement that restorers could sketch it first and save its 16th-century frescoes.

"Restorers showed up for work and saw that their measurements and most of the frescoes were buried in the rubble," said historian Ratomskaya.

Putin's initial idea was to demolish the 14th Wing and rebuild the convent and monastery, but experts are advising against this.

"Those stones had seen history. The (convent and monastery) are an enormous loss, but it's wrong to think they can be replaced by life-size models," Ratomskaya said.

"Once a mistake is made, you cannot undo it," she said. 

Asked whether a decision has been made to reconstruct the monastery and convent, Putin's spokesman Dmitry Peskov said: "The issue is not currently on the agenda" with no construction planned there for the time being.

- 2,000 treasures -Archaeologists have rejoiced at the opportunity to dig in the area where the first Muscovites settled in the mid-12th century, though Makarov admits he had reservations.

"At first I wasn't sure it would be a good thing (to demolish the Stalin-era building) but now I like the view," he said, looking over the empty space, which for the moment is a grassy park open to visitors. 

"That feeling of a cramped stony dungeon is gone, the Kremlin has become more open."

Among more than 2,000 treasures unearthed so far from underneath the former Chudov monastery is a stone casting mould with Moscow's first-known Cyrillic writing.

There is also a sooty layer that is likely to be evidence of fire in the city set alight by Mongol ruler Batu Khan in the 13th century as his Mongol state, known as the Golden Horde, invaded Europe.

A few centuries later, Chudov cathedral's altar served as chambers for Napoleon's "Iron Marshal" Louis-Nicolas Davout during the 1812 occupation of Moscow.

After the French army retreated, it had to be reconsecrated.

- Reused cathedral stones -Putin in August ordered the creation of an archaeological museum at the site of the underground dig.

Makarov envisages a place that evokes the tragedies of Kremlin's past by keeping the 14th Wing's basement intact.

"Here there are Constructivist-style columns from 1932 and their base is made of monastery stones -- stones from the old cathedrals that were reused, even tombstones," he said.

"I think we should show that juxtaposition," he said. 

"We should show the paradoxes and fractures of history."