Historic stones taken from Cardiff Castle have appeared for sale on eBay - with bids starting from £300.
The stones, advertised as "11th Century limestone rocks", were pinched from the Norman shell tower inside the castle by a tourist in May last year.
The eBay listing - which has since been taken down - said the rocks were "a fantastic opportunity for someone who is thrilled to own a piece of history".
Whoever buys the rocks "must have an interest in historical artefacts," said the listing, which said the pieces of the castle are currently located in Frenchay, Bristol.
Staff at the castle tweeted: "We have contacted the seller on eBay to request these items are returned, otherwise we'll be asking South Wales Police for some assistance.
"We're busy calculating the Keep's worth at £300 per brick."
But the seller said they had never intended to make a profit on the stones.
Speaking anonymously they said: "I entered the Norman Shell tower inside the castle, while I was looking inside and having a walk, I decided that I wanted to take something from this castle as a memory.
"I looked at the ground, saw some bits decaying and falling pieces of rock and took them with me. Anyone can really take anything on the ground.
"But I kinda wanted to make a value out of these rocks as being from inside the castle.
"Many people told me that this is just silly and they are rocks at the end of the day.
"I wanted to prove them wrong and see if they could actually get a value if I listed them on eBay since many foolish things gets listed on ebay and they sell for thousands."
The seller said the rocks "are less than a hand fist size" but they would be happy to return them to the castle if asked.
They said: "There is no intention for selling them really. The real purpose for listing them on eBay is to see whether they have any value or whether people will be willing to buy them.
"At the end of the day, I intend to keep them for myself as a memory whenever this is possible.
"I would be happy to return them back.
"However, provided they arrived back to the castle. I think the first thing that would go on the administration's mind when opening the tiny parcel is 'what a waste of our valuable time.'
"These are simply bits of rocks found on the ground. I even found some vegetation on them and had to clean them with some tissues.
"The big difference probably is that they were found inside the castle and they where part of the structure of the ancient tower, at least as I assumed the moment i picked them up.
"You can't really distinguish them from other ordinary rocks if they where found anywhere else."
Cardiff Castle is a medieval castle and Victorian Gothic revival mansion.
The original motte and bailey castle was built in the late 11th century by Norman invaders on top of a 3rd-century Roman fort.
Cardiff Castle has demanded the historic stone is returned and joked that if one stone is worth £300 the entire castle must be worth a fortune.
A spokesman for Cardiff council, which owns the castle, said: “Cardiff Castle is a scheduled ancient monument and one of the most significant historic buildings in Cardiff. Conserving the building for residents, visitors and future generations to enjoy is essential, so the suggestion that rocks have been taken from the site is clearly concerning.
“We would like to remind people that removal of any part of the building is a criminal offence and appropriate action will be taken if necessary.”
