Ever since it was built by Ivan the Terrible, St Basil's Cathedral with its host of multi-coloured domes has been one of the most recognisable buildings in all of Russia.
Today it is swathed in scaffolding and green dust-netting. Builders shout to each other, as they winch materials to the top of one St Basil's towers.
"It's like a visual icon of Red Square. I think it's beautiful," says an American tourist gazing up at the building.
A man next to her adds: "Growing up as a child with the Cold War, whenever we got to see any news footage of Moscow, of the military might of the USSR, it always seemed this building was looming large in the background."
Military parades have taken their toll on the old building
A three-year restoration project on the cathedral's exterior is just coming to an end. Its onion domes sport gleaming new paintwork, swirls of red and white, chequered patterns in green and yellow.
But inside, the walls - built over 400 years ago - are slowly cracking. There are fears Russia is in danger of losing its most famous landmark.
If you walk up the dark and narrow staircase in the centre of St Basil's, you emerge in one of the nine separate chapels inside the building. The walls are covered in old paintings and frescoes.
Shafts of light stream in from windows high in the walls. But long cracks run through the brickwork.
A report commissioned by the Russian Government has said the cathedral is slowly sinking into the ground and cracking apart. It warns St Basil's could fall into ruin if its foundations are not strengthened.
Tatyana Nikitina is the chief architect for the restoration.
"The earth under the cathedral is more solid in some places than others," she says.
"As a result the cathedral has settled unevenly over the years. So that's why we get these cracks."
The use of Red Square for huge public events has also helped weaken St Basil's foundations. During the Soviet years massive military parades were held here. Squadrons of tanks and missiles launchers would grind their way past the cathedral.
In more recent years it has been used as the venue for rock concerts by the likes of Paul McCartney.
"When tanks used to cross Red Square we could feel everything shake," says Igor Mitichkin, the man responsible for the upkeep of all the monuments around the Kremlin.
"Thank God there aren't tanks any more. Gun salutes don't really have any effect on the buildings. But rock concerts, if they are too loud, they do."
Right across the road from St Basil's is Number Five, Red Square. It is a long cream-coloured building with a green tiled roof, and a commanding view of the cathedral and the Kremlin.
There are plans to turn it into a giant hotel complex with an underground car park big enough for 600 vehicles.
The cathedral's curator, Lybov Uspenskaya, says she has not been consulted about the plans for the hotel so has no way of knowing if they might further undermine St Basil's.
Rock concerts can also get the cathedral's walls shaking
"Nobody has showed us any plans for the development. As for who's in charge of it, they have nothing to do with us. We don't even know which authorities are handling the project."
Mrs Uspenskaya is not going to question the new development. It is not her place to do so she says. Instead she will wait and then fix any new damage that is done to cathedral.
But she insists St Basil's will still be standing as a symbol of Russia for centuries to come.