I see that Transport for Wales (Welsh Train Company) have a 2 for 1 deal for entry to CADW Castles. (CADW means castles under the control of the Welsh Government). So, be sure to retain your old ticket as proof of travel by train. Note that if you put it into a ticket barrier slot that you won’t see it again so see a member of staff instead and they will open the gate.
https://tfw.wales/ways-to-travel/rail/savings-and-offers/cadw
The CADW is more than just castles. The organization maintains historic properties in Wales. Some you must pay to enter, but many are free. If you plan to spend much time in Wales, consider buying the annual pass.
Cadw is actually a welsh noun which translated into English means to 'keep' or 'preserve'.
The welsh equivalent of English Heritage, with an equally broad remit.
It is not an acronym and can and does relate to anything heritage, including but not limited to castles