Archaeologists find vast medieval palace buried under prehistoric fortress at Old SarumNow, just think about what is being claimed here. Archaeology proceeds from the assumption that material is deposited chronologically. The further down you go, the earlier the material. Whenever find a building below another building, the earlier one is under the later one.
If the building at Old Sarum was really built by Henry I in the twelfth century then it is certainly medieval. It is not prehistoric. Prehistoric England is earlier than medieval England. By the time of Henry I, England had definitely entered its historical period.
If archaeologists actually had found a medieval palace under a prehistoric fortress, then we might seriously think about space aliens being involved. If you read the story, you will find that such is not the case.
To his credit, David Keys, who wrote the story, did not make the mistake; whoever wrote the headline did. By writing "under" instead of "inside" the editor wins the prize for the silliest archaeological story of the season.