Hillforts in Lithuania

The Lithuanian word for hill fort is piliakalnis (plural piliakalniai), from pilis (=castle) and kalnas (=mountain, hill).
Lithuania has hill forts dating from the Bronze Age in the 1st millennium BC. The earliest examples in present day Lithuania are found in the east of the country. Most of these forts were built or expanded between the fifth and fifteenth centuries, when they were used in the Dukes' Wars, and against the invasion of Teutonic Knights from the west. Most forts were located on the banks of a river, or a confluence where two rivers met. These fortifications were typically wooden, although some had additional stone or brick walls. The hill was usually sculpted for defensive purposes, with the top flattened and the natural slopes made steeper for defense.
During the early years of Grand Duchy of Lithuania piliakalniai played a major role in conflicts with the Livonian Order and the Teutonic Knights. During this period the number of piliakalniai decreased, but those that remained had stronger fortifications. Two main defense lines developed: one along the Neman River (against the Teutonic Order) and another along the border with Livonia. Two other lines started to form, but did not fully develop. One was to protect Vilnius, the capital, and the other line in Samogitia, was a major target for both orders. This territory separated the two Orders and prevented joint action between them and Pagan Lithuania.
Most of the forts were constructed of wood and were quite easy to burn. As firearms and artillery developed, piliakalnis and their castles became ineffective. Also, the Livonian Order was defeated in 1236 in the Battle of Saule. The Teutonic Knights suffered a major defeat in 1410 in the Battle of Grunwald and did not pose any further major threat.
According to the Lietuvos piliakalnių atlasas (English: Atlas of Piliakalniai in Lithuania), there were 826 piliakalniai in Lithuania. Some researchers present a total number of 840 known piliakalnis in 2007; the number is likely to increase as even more of them are discovered every year. Most piliakalniai are located near rivers and are endangered by erosion: many have partly collapsed as the flooded river has washed out the base of the hill. Now around 80 percent of piliakalniai are covered by forests and are hardly accessible to visitors.