Celtic circular wall of Otzenhausen The Celtic circular fort at Otzenhausen is one of the biggest fortifications the Celts ever constructed . It was built by Gauls of the Treveri tribe , who lived in the region north of the fort . The fort is located on top of the " Dollberg " , a hill near Otzenhausen in Germany , about 695 m above sea level . The only visible remains are two circular earth ramparts , covered with stones . History . In times of war , the circular rampart was a strong fortification against enemies . Theories suggest this one might have been more than just a refuge . There might have been a permanent settlement , a village or the seat of a local leader . The first fortification was constructed in the 5th or 4th century BC , but the real heyday of construction dates to the 2nd and 1st century BC . For reasons yet unknown , the fort was abandoned shortly after this expansion . Description . The site is formed in the shape of a triangle with rounded ends . One rampart surrounds the whole fort . On the southern side , another similar embankment is built about 40 metres in front of the main one . The ends of this outer rampart approach the main one but do not touch it . Because the entrance of the main rampart is located on the western side , no significant purpose for the outer one has been determined . From west to east the fort extends 460 m , from north to south 647 m . The total length of the ramparts is 2500 metres and they contain 240 , 000 cubic metres of stone . Thousands of beams were attached to the ramparts which , as the diagram shows , probably presented a vertical stone wall to the exterior . Julius Caesar describes such ramparts in Book seven chapter 23 of his De Bello Gallico . " But this is usually the form of all the Gallic walls . Straight beams , connected lengthwise and two feet distant from each other at equal intervals , are placed together on the ground ; these are mortised on the inside , and covered with plenty of earth . But the intervals which we have mentioned , are closed up in front by large stones . These being thus laid and cemented together , another row is added above , in such a manner that the same interval may be observed , and that the beams may not touch one another , but equal spaces intervening , each row of beams is kept firmly in its place by a row of stones . In this manner the whole wall is consolidated , until the regular height of the wall be completed . This work , with respect to appearance and variety , is not unsightly , owing to the alternate rows of beams and stones , which preserve their order in right lines ; and , besides , it possesses great advantages as regards utility and the defence of cities ; for the stone protects it from fire , and the wood from the battering ram , since it ( the wood ) being mortised in the inside with rows of beams , generally forty feet each in length , can neither be broken through nor torn asunder . " On the western side , there was a gateway , build of wood , standing on eight posts . This gateway was double - winged and 6 m wide . A central post divided the gateway into two gates of 2 . 5 m each . The wooden posts sunk into the holes of the gatepost were wedged with stones . Broken rock on the ground prevented the earth from being washed away . Perhaps there was a second gate on the eastern side , but its existence is not yet proven . From other examples of Celtic forts it is known , that the gateway presumably was a canopied wooden battlement . Because this gateway is build to the inner side of the wall , the ends of the wall and the battlement form a small square , which can be attacked from three sides without leaving cover . Important for the use of the fort is the existence of a spring on the top of the Dollberg . This spring is a result of the impermeable quartzite the ground consists of . Holes in the ground , sockets for the wooden posts , prove the existence of buildings . It is unknown whether their purpose was accommodation or storage . Aftermath . In roman time a small temple of 2 . 15 metres x 2 . 70 metres , dating from the 2nd or 3rd century A . D . was built of quartzite rubble stone and brick - shaped sandstone . When German tribes invaded the Roman territories in 4th century A . D , the fort was used again . During the Thirty Years War ( 1618 – 1648 ) the citizens of the surrounding villages again took refuge in the remains of the Celtic fort . The first documented appearance of the circular ramparts happened within the Grimburger Salbuch , a geological register of 1589 , the first picture of the enclosure in the Gazette de Metz from 1836 . The same year , Count Villers von Burgesch addressed a petition to the Prussian king Friedrich Wilhelm III to bar the inhabitants of the nearby villages to carry off stones for use as building material . Reacting to this petition the Crown Prince later King Friedrich Wilhelm IV visited the fort in 1837 . File : Ringwall von Otzenhausen . jpg Excavations took place in 1883 and from 1936 to 1939 . Present . The site can be visited all - season , it is not accessible by handicapped persons . Three information paths lead the visitor through the fort : Since 1999 the Terrex gGmbH organizes excavations on and around the fortification . Today the whole area with exception of the stone circles is covered with forest .