Admission is free to Caerphilly Castle all day on Tuesday March 1, with refreshments on sale and a Welsh craft fair in the Great Hall.
The Castle joins 13 other Cadw sites across Wales in opening their doors for free to celebrate St David’s day.
Ken Skates, Deputy Minister for Culture, Sport and Tourism, said: “Wales is a country teeming with fascinating heritage, from its historic buildings to its iconic stories. It is extremely important that we ensure our heritage sites are accessible to everyone inside and outside of Wales to explore and enjoy.
“By opening sites for free, we’re able to give people across Wales the chance to explore their local monuments and learn more about their heritage in honour of our Patron Saint.”
In the evening, the Great Hall will host a ticketed St David’s Day concert from 7pm to 9pm, featuring readings from the works of Welsh poets such as Roald Dahl and Dylan Thomas, and traditional Welsh songs from the Aber Valley Male Voice Choir.
Tickets cost £10. Under 16s must be accompanied by an adult.