The popular attraction has been closed to the public since it was damaged by a fallen tree during Storm Freya in March.
The works are being carried out by the borough council in association with English Heritage and are expected to take four to six weeks.
These include the replacement of fencing and damaged tarmac on the footpath that winds up to the top of the mound.
The oak tree, which was believed to be around 100 years old, came down in the high winds.
It was found to be suffering from a fungal infection. Another tree was also deemed to pose a health and safety risk.
A grant of £20,231 was secured from the Government’s Local Authorities Parks Improvement Fund in order to finance the renovation of the Motte.
The feature commands views over the town and forms part of the audio tour of the Castle.
The adjacent Castle Lawn was the venue for a new music festival in the town over the first May Bank Holiday weekend.
It will be also hosting the town’s Food & Drink Festival on May 25-26 and the fete after the Tonbridge Carnival on June 16, as well as other events throughout the summer.