Mlata has the oldest free-standing megalithic buildings in the world: Temples, constructed about 6000 years ago by a culture without weapons and warfare and used well over 1200 years. These structures are of a beautiful symmetry, with walls of regular dressings, made up of HUGE stone slabs (nobody knows how they transported them), with unexplainable symbols and altars.
Models of the temples have been found, indicating that prehistoric people could think and plan ahead. Other things have been found that neither archeologists, nor tourists, nor Erich van Däniken can explain.
Some temples have been used by later civilizations, alternately as burial grounds, cult places or quarries. The ruins of the temples are still there, thousands of years later, and if they are not used as tourist attraction they pretty much get into the way of contemporary urban planning.
How where they built?
Why were they built?
Who were the priests?
What happened inside?
Were aliens involved?
Are they oriented according to the solstices?
The archeologists face the constant danger of over-interpretation, of reading more into the evidence than what was actually intended by those who left it.
Summing up: we really have no clue.