Trelleborg was another location on our folks trip. Founded around 1000 A.D. (980 A.D. according to dendrochronology) . Trelleborg is a collective name for 6 Norse era “castles”. Trelleborg, Viking ring fortresses, were of stricktly circular shape (“perfectly circular with gates opening to the four corners of the earth, and a courtyard divided into four areas which held large houses set in a square pattern”). Unlike other ring castles from the period the ring castles which follow the Trelleborg model are constructed after a strictly geometrical plan and measured with the Roman foot. The pointed bottoms of the moats is another element borrowed from the Romans.
Trelleborg at Slagelse was designed as an exact circle with two roads that crossed at right angles in the geometric center and led to the four gates with two always opposite to each other. In each of the four quarters stood four almost identical longhouses arranged in a square. Unlike the others, the fortress was extended with a sort of bailey. The whole fortress may have supplied room for some 1,300 people [Wikipedia].
As a museum it’s definitely worth visiting! It’s location is [here].