Work begins this week to rebuild the south-facing terrace at Nottingham Castle destroyed nine years ago.
Work starts to repair damage caused in 1996
A rock-fall saw 200 tons of rubble, mud and infill material collapse after a leak in a water main.
The six-month project will see a new stone wall anchored into the base of the rock to support a concrete terrace.
That will be supported by concrete beams allowing access to a medieval wall of the original castle uncovered in the 1996 landslide on Christmas Day.
The reinstated sandstone parapet will include a glazed section so that people who cannot see over the top will still be able to enjoy the castle's panoramic views over south Nottinghamshire.
Councillor Leon Unczur, city council execute member for culture, community services and tourism, said Castle Rock is a Grade II scheduled ancient monument.
He said: "We have had to take a step by step approach in all the decisions after the landslide because the site is so sensitive historically and the nature of the work is technically complex."