This marks the end of the Welsh government's Open Doors 2015 event.
Caernarfon Castle, Gwynedd, Ruthin Jail, Denbighshire, Caerphilly Castle, Strata Florida Abbey, Ceredigion, and St David's Bishop's Palace, Pembrokeshire, are all involved.
Deputy Tourism Minister Ken Skates said he hoped it had sparked an interest to explore "our fascinating heritage".
Heritage sites across Wales have free entry this weekend as part of a scheme to get people interested in history.