I visited the site in 1992. It's fun but there is absolutely no true evidence whatever that this is a prehistoric site, or anything more than a farmer trying to find something useful to do with what was coming out of his field in the rocky soil of a New Hampshire mountaintop. The proprietors (who make a fine living from this tourist destination) have found items that carbon dated to a very long time back, but there was nothing to connect the structure itself to the carbon dated items. The proprietors don't really like to allow outside investigators to dig, because they always come back with answers that the proprietors don't want to hear. The site is disturbed and almost not worth the effort. The two biggest arguments for declaring this anything more than a hoax or misguided recognition of an ancient site are 1) why would anyone in the colonial era go to the bother to build a hoax with stones of several tons; 2) archeologists have found evidence in the last decade of human presence in North America as long ago as 50,000 years.
Something is not right. This message is just to keep things from messing up down the road