Windsor Castle's Round Tower is to open to the public for the first time in almost four decades.
Visitors to the 800-year-old Berkshire landmark will be able to walk up its 200 steps to take in views of the town and the London skyline.
Work to combat subsidence closed the doors of the tower to visitors in 1975 which saw the building under-pinned.
The Queen usually spends her weekends at the castle and during Easter takes up official residence.
The tower is part of a complex of buildings that make up the castle site and houses the Royal Archives.
The Round Tower was built in 1170 by Henry II with heath stone from nearby Bagshot.
It replaced a wooden Norman keep which was part of the Windsor Castle constructed by William the Conqueror from 1070-86.
Sitting on an artificial chalk mound (motte) made from the spoil thrown up when a ditch was dug around the fortress, its appearance dates from George IV's major remodelling of Windsor in the 1820s.
The top of the tower is 40 metres above the town of Windsor.
Up to seven million visitors a year visit Windsor, with tourism across the Royal Borough bringing in an estimated £500m to the local economy.
Emma Shaw, of the Royal Collection, said it continued to look at ways of adding attractions to all the official residences of the Queen.
"In recent years we have introduced the kitchen tours and the one year pass to encourage return visits. This tour is the next step in this on going process."
According to BBC News