It will be the first time the landmark has hosted a boxing event, though various pop concerts have been held there.
Calzaghe, who has held the title since 1997, won his 10th title defence against former International Boxing Federation champion Charles Brewer in Cardiff on 20 April.
He said: "Come August 17, I will be king of this castle.
"I promise everyone I'm going to repay them with a nice, clean knockout on the night. I'm feeling good."
Calzaghe added: "You cannot beat the Welsh fans for the support and passion that they show to their sporting heroes. They are absolutely tremendous.
"They were fantastic in my last fight against Brewer and I want to give them something to remember.
"What can be better than defending my crown at Cardiff Castle? It is a venue steeped in history and is world renowned.
"It will be an unforgettable night."
The Prince of Wales will receive an invite to sit ringside on the night and Calzaghe added: "At such a special occasion as this, I hope he will be able to make it on the night."
Promoter Frank Warren said that 30-year-old Calzaghe would face a 'top-ranked challenger, to be named shortly'.
But he admitted that World Boxing Council champion Eric Lucas was one of the names in the frame.
Lucas last defended his WBC title with a unanimous win over Vinny Paz in Mashantucket, Connecticut, on 2 March.
One leading candidate, America's Dana Rosenblatt, ruled himself out when he was badly cut in a bout in Boston last weekend.
Calzaghe added: "Whoever it is, I'm not going to make it as hard for myself as in April, when I had to go the full 12 rounds to beat Charles Brewer.
"I hope I can make it nice and short this time around."
Two other world title fights will take place on the Calzaghe bill.
WBU light-middleweight ruler Takaloo faces his WBO counterpart, Daniel Santos, in a unification bout.
And WBO super-bantam king Agapito Sanchez, from the Dominican Republic, will face fellow countryman Joan Guzman.
British welterweight champion Neil Sinclair will also feature in a non-title bout against imported opposition.