The Pre-Roman Iron Age began in Estonia about 500 BC and lasted until the middle of the 1st century AD. The oldest iron items were imported, although since the 1st century iron was smelted from local marsh and lake ore. Settlement sites were located mostly in places that offered natural protection. Fortresses were built, although used temporarily. The appearance of square Celtic fields surrounded by enclosures in Estonia date from the Pre-Roman Iron Age. The majority of stones with man-made indents, which presumably were connected with magic designed to increase crop fertility, date from this period. A new type of grave, quadrangular burial mounds, began to develop. Burial traditions show the clear beginning of social stratification.