Portugal is the site of many ancient dolmens constructed thousands of years ago. Dolmens — called anta in Portugal — are stone structures built by Neolithic peoples undergoing a megalithic phase. The best-known megaliths include Stonehenge, but structures made of stone slabs occur all over the world and have been erected in historic times by some peoples.
The Portugese anta are dolmens of the type known as passage tombs, although not all of them may have served a burial function. In fact, no one knows exactly what the purpose of these structures were and they may have served different roles in different places at different times.
The chapel of St. Denis in Pavia is one of many anta that were converted for Christian use. This one was made into a chapel in 1625. Here is a late example of the practice of Christian missions in Europe repurposing an ancient holy place. Pope Gregory the Great explicitly lays out this policy in a letter of 601 AD recorded by the venerable Bede in his History of the English Church and People:
…we have been giving careful thought to the affairs of the English, and have come to the conclusion that the temples of the idols among that people should on no account be destroyed. The idols are to be destroyed, but the temples themselves are to be aspersed with holy water, altars to be set up in them, and relics deposited there. (Bede’s History, translation by Leo Shereley-Price)
This is pretty much a reversal of an earlier letter of Gregory’s (also recorded by Bede) in which he tells King Ethelbert to “suppress the worship of idols and destroy their shrines”. But, upon careful thought, Gregory has determined that it is best to have the people worship at their customary places.
They are no longer to sacrifice beasts to the Devil, but they may kill them for food to the praise of God, and give thanks to the Giver of all gifts for the plenty they enjoy. If the people are allowed some wordly pleasures in this way, they more readily come to desire the joys of the spirit. For it is certainly impossible to eradicate all errors from obstinate minds at one stroke, and whoever wishes to climb to a mountain top climbs gradually step by step, and not in one leap.
So, a thousand years after Gregory’s letter, the chapel at Pavia became one more step toward the mountain top.
Note: good photos of the chapel may be found here.