Caernarfon Castle will undergo a £4m revamp to open up previously unseen areas of the castle and build a cafe.
Gwynedd Council on Thursday approved the plans, which also include a lift to make the castle more accessible.
The 13th Century castle, which hosted the Prince of Wales' investiture in 1969, is a World Heritage Site.
Deputy Minister for Culture, Sport and Tourism Lord Elis-Thomas said it was vital to make Wales' past more accessible.
The project is set to be one of Cadw's largest and most complex to date - with plans for vital conservation work to help protect its medieval structure, accessibility improvements and the introduction of new and more interactive spaces.
Accessible toilets, a gift shop and a cafe will be some of the features added, according to the Local Democracy Reporting Service.
Work will begin on 30 November 2020 by erecting 190 tonnes of scaffolding to gain access to the top of the 25m-high King's Gate towers and should be completed by early-2022.
Director at Grosvenor Construction, Will Mellor, who is overseeing the project, said he felt "honoured" to be working on the project and he "looks forward playing a valuable part in the castle's 700-year long history".
Lord Elis-Thomas, said he "greatly welcomed" the project as it "marks a positive new chapter for Caernarfon Castle and its visitors".
"Making Wales's past more accessible to people from all walks of life is vital, and through innovative technology, we can unlock unexplored areas of our historic sites while delving deeper into their histories," he added.