Tower Two lies between Bitchdaughter Tower and Baile Hill in the south-west corner of the city walls.
City of York Council stonemasons have now stabilised the tower. They removed rubble which was put inside the structure in the nineteenth century to create the existing paved walkway but had been pressing against the tower wall.
A new walkway will now be created, which has been designed not to press against the tower wall and will also allow walkers to see inside.
Tower Two is believed to date back to the mid-fourteenth century and archaeologists hope the work will help them learn more about the abandoned castle at Baile Hill.
Dr Louisa Hood, walls manager at the council, said stonemasons have dismantled part of the tower parapet and are now working their way down the cracks in the tower walls.
She said: “Each stone that has been removed has been labelled so that when the parapet is rebuilt, it will be returned to the same place.
“This is a rare opportunity to understand exactly how the tower was built and we’ve spotted some interesting revelations.
“Apart from the parapet, most of tower appears to be original medieval construction, which we’ve not been able to identify before now because of repair works over time, with plenty of previous grouting and repointing masking the original build.
“Not many people think about what’s under the foundations of the stone city walls that we see today, but it’s fantastic to see through the exposed walls that we can now identify its construction date of the 1330s.”
The work will take at least four months but people will still be able to walk around the walls while repairs take place thanks to a temporary walkway.
Baile Hill is where William the Conqueror built a motte and bailey castle to guard the River Ouse after he seized York in 1068.
Bitchdaughter Tower was the king’s prison in 1451, according to a report by Friends of York Walls.
It says the name is believed to be derived from the old French word “dortour” which does not refer to a woman but to a bedroom and that the tower was named as such because it was considered a nightmare place in which to be incarcerated.
Once work at Tower Two is completed, the internal wall will be visible to visitors from the new walkway.
Ian Milsted at York Archaeological Trust: “Exploring the city walls is rightly popular amongst locals and visitors, the new walkway will give us all an opportunity to better understand this historical monument.
“This will be the first time in 200 years that the inside of this tower will be exposed, and with the infill removed, more of the historic tower will be visible for generations to come - exposing the past to preserve its future.”
Coun Keith Aspden, council leader, said: “York’s city walls are the most complete and finest in England, making them one of our most treasured historical assets. That is why it is incredibly important that we continue to maintain and repair the walls, so residents and visitors can enjoy them for many years to come.
“The original walls were built by the Roman’s in 71AD and by carrying out this project, we will better understand the history of the tower and preserve it for future generations.”