The most famous prehistoric structure in Europe, possibly the world, Stonehenge stands on Salisbury Plain, an area rich in monuments such as long barrows and round barrows. It draws visitors from all over the world but viewing is restricted and it is difficult to get a sense of the grandeur of the place amongst all of the tourists.
There have been many timescales attached to the building of the site although it is generally held to have been completed in  3 identifiable phases over a 1700 year period starting perhaps around 3200BC.
Stonehenge I
Begun in the Neolithic, a circular bank nearly 2 metres high was built with a quarry ditch outside and a smaller bank around the edge of the ditch. This henge structure was over 85 metres wide with an entrance to the South and one at the Northeast which may have been some kind of ceremonial gate. Set beyond this entrance was a huge sarsen block known as the 'Heel Stone' which was aligned on the midpoint of the northern moon's major and minor rising. Inside the henge some kind of wooden structure was built, possibly a timber circle or a mortuary house. Later 56 pits were cut inside the bank - the 'Aubrey Holes', but these seem to have been refilled shortly afterwards, some contained cremated human remains.
Stonehenge II
Begun sometime around 2500BC the second phase saw two concentric stone circles added inside the bank. These 'bluestones', some 80 in all, were brought from the Preseli mountains in Wales (possibly from Carn Meini) over 135 miles away. They consisted mainly of diorite and some of rhyolite, both types of volcanic rock, weighing up to 4 tons each. It is believed that they would have traveled from the mountains to Milford Haven where they were loaded onto rafts and brought along the Welsh coast to the Severn estuary, then along the Bristol Avon and the River Frome, via the Wiltshire Avon before being offloaded to the banks of the river. From here they were brought to the sanctuary, perhaps involving some kind of ceremony and finally finished their incredible 250 mile journey. Another four blocks, known as the 'Station Stones' were also erected around the circle - only two now remain. The Stonehenge Avenue is thought to have been built during this time - a pair of parallel banks with outer ditches that emerge from the North East entrance through the Heel Stone and run down to the River Avon, it could be that the avenue was constructed to commemorate the route of the bluestones.
Stonehenge III
It appears that the second phase was never completed and the site remained unchanged until about 2300BC at the dawn of the Bronze Age when a third phase of building and remodeling began with the bringing of sarsens from the Marlborough Downs, 25 miles to the North. Roughly cut to shape before moving, these 80 huge blocks weighing between 20 and 50 tons each were placed on sledges and dragged over rollers to Stonehenge. It is estimated that it would have taken a team of 1000 men 7 weeks to move each stone, the whole mammoth task requiring over 10 years to complete.
Once at the site, the rough sarsen blocks would have been shaped by a team of masons, another task that would have taken several years. At this time the bluestone rings had been taken out and the ground leveled ready for the 30 uprights that formed the great outer circle. Pits of about 1.5 metres deep were dug and the blocks tipped into them, then pulled upright and their tops leveled, but with a protruding tenon to receive the lintels. These lintels had mortises on their undersides to fit the uprights and tongue-and-groove joints to bind them to their neighbours. Inside this circle were raised the 5 great trilithons in a horseshoe shape with the opening facing the entrance, each consisting of 2 huge mortised uprights, between 6 and 7 metres tall with a single tenoned lintel and with the Altar Stone at their centre. Of the bluestones, 22 were now placed inside the trilithons, then moved to form another simpler horseshoe structure. Others were placed in a circle outside of the trilithons. Some time later the 'Altar Stone', a block of micaceous sandstone, was brought from South Wales and erected within the horseshoe. Two stones were put up within the Northeast entrance to frame the midsummer sunrise, of which one remains as the 'Slaughter Stone'

There have of course been many theories advanced over the years to explain the purpose of Stonehenge, from the plausible to the downright eccentric. It may be that it is closely related with the changing of the seasons and astronomical phenomenon such as the rising and setting of the sun and moon. It is likely that no-one will ever come up with a definitive answer to this prehistoric enigma.