Stonehenge
Stonehenge is located on the Salisbury Plain. It consists of 4 courses of stones that are raised inside a circle formed by a concentric bank and ditch. Stonehenge is different from other henges in that the bank is located inside the ditch rather than the other way round. It was constructed not as a single event but a series of events during the Neolithic and early Bronze Age. There is no archaeological support that Merlin constructed Stonehenge.
To the contrary, the last building event at Stonehenge was completed at least 1000 years before Arthur's time. There are no mass burials at Stonehenge, and no abbey ever stood near the ruins. Part of the legend is that Caliburn, the sword the young Arthur pulled from the stone, was forged from a sarcen that came from Stonehenge or Avebury. Avebury is roughly contemporaneous with Stonehenge and has an impressive (and typical) bank and ditch perimeter with 100 sarcen stones arranged in an inner circle. The town of Avebury was built inside the stone circle. The sarcen stone used to forge Caliburn may have had magical significance to the Celts.
The final Battle of Camlann, in which Arthur was mortally wounded, is located by some historians as being fought on the Salisbury Plain. Excalibur may have been thrown into the confluence of the Avon, Bourne and Nadder at the site of present-day Salisbury. Other legends suggest that Excalibur was thrown into Loe Pool in southern Cornwall. Giving offerings to water deities was a common Celtic practice.