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".

Philip Blackwood was jailed last March for using an image of Buddha to promote his bar.
Those being released include 52 people jailed for political offences.
It comes just over a week before the new parliament meets for the first time since last year's election victory by the opposition pro-democracy NLD party.
Sentences for nearly 80 prisoners on death row have also been commuted to life in prison.
Mr Blackwood's sister, Angela Blackwood, said the family had been told that he would be released in the next few days "or it could be up to a week".
It was not immediately clear if two Burmese men jailed with him were also being released.
Mr Blackwood had posted a picture on social media of Buddha wearing headphones to promote a cheap drinks event at the Yangon bar he managed.
Following complaints from monks, he was sentenced to two and a half years with hard labour for offending the Buddhist religion.
The BBC's Jonah Fisher in Myanmar says that despite the latest releases, the country's outdated legal system - administered by often vindictive local officials - is still jailing people for political activities, usually unauthorised protests.
On Friday, an ethnic Kachin activist named Patrick Khun Jarli was jailed for six months for posting a picture online that mocked the military.
Amnesty International said the sentence was "a stark reminder of how prevalent repression still is" in Myanmar, and said nearly 100 other prisoners of conscience remained behind bars.
Presidential spokesman Zaw Htay said on Facebook that a total of 102 prisoners were being released.
One official said 21 of them were being released from the notorious Insein prison, which has long been used for political detainees, in Yangon, also known as Rangoon.
Its most famous inmate was Aung San Suu Kyi, the pro-democracy leader whose party won the historic elections in November, ending decades of military or military-backed rule.
Myanmar's former military government had been accused of wrongfully imprisoning about 2,000 political opponents, dissidents and journalists over the years.
But since 2010, as it began edging towards democracy, it has freed hundreds of detainees.
Heritage sites across Wales have free entry this weekend as part of a scheme to get people interested in history.
[NEXT_CONCEPT]
A New Zealand man serving a two-year sentence for insulting Buddhism is among more than 100 prisoners to be released from jail in Myanmar.