Oakham Castle, in Rutland, dates back to 1180 but only the Great Hall is currently visible.
As part of a £2m restoration, trees and shrubs have been removed from the embankments and the remaining sections of ancient walls uncovered.
Work also includes retiling the hall’s roof, plastering its walls and a new toilet block.
Oakham Castle was originally built as a manor house and was later heavily fortified with walls, a moat and a drawbridge but by the 16th Century most of the castle was a ruin.
Sections of the defensive curtain wall have survived underneath mounds of earth but are on Historic England’s “heritage at risk” register.
County councillor Roger Begy, culture portfolio holder, said: “Conservation experts have already taken steps to preserve the castle boundaries but this is the first time that we’ve dug down to expose the walls themselves.
“Once the restoration is complete we hope to have repaired and revealed much more of the defensive curtain wall so that visitors can get a true sense of what this incredible building was like many hundreds of years ago.”
Inside the Great Hall, more than 200 commemorative horseshoes given by visiting monarchs and peers and dating back 550 years are being cleaned.
The Heritage Lottery grant is also paying for lime render on the hall’s walls, new tiles on the roof, a toilet block and wheelchair access ramps.