Cardiff Castle is celebrating 60 years since it was handed to the city by the 5th Marquess of Bute.
Cardiff Castle and Bute Park were given to the city in September 1947
A 1940s theme day includes free entry to the grounds and staff dressed in period clothes.
The programme also includes guided tours of hidden tunnels used as air raid shelters during World War II.
The site of the castle site was originally a Roman fort in the 1st Century AD and passed to the Bute family in 1776 through marriage.
The Butes helped to lay down the city's infrastructure and in 1947, the then marquess presented the people of the city with Cardiff Castle and adjoining Bute Park, which are both now managed by the county council.
The day of celebrations marking the 60 years since includes a sing-a-long session of 1940s songs as well as an exhibition of vintage cars.
Castle manager Kevin Burt said: "The Butes actually come from the Stuart line, the royal Scottish family, and were a massive aristocratic land-owning family and, by marriage, owned pretty much all of Glamorgan.
"It was the second marquess who basically turned Cardiff from a sleepy very small town into the massive port that it became, by building the docks.
"Anywhere you go in Cardiff you will see the names of the Bute family.
The road outside Cardiff Castle is one of the busiest in the city centre
"Ninian Park (Cardiff City FC's ground) is named after Ninian, one of the sons, Colum Road, another one of the sons. Their names are everywhere."
In 2003, the castle was the focus of a £6m lottery-funded renovation effort, which was then Wales's largest conservation project. It included re-pointing the walls and opening up the hidden tunnels.
Previous contractors at the castle include famed landscape artist Capability Brown, who re-worked the gardens early in the Bute era.
After the Romans, the Normans created a motte and bailey structure on the remains of their fortifications, and it was strengthened in the 13th Century after being damaged in a Welsh revolt.
The castle was held by the Despenser family during Owain Glyndwr's 15th Century rebellion, but fell to Cromwell's army during the English Civil War.