Castella recens facta
Ring fortress (UK) or ringwalburg (NL), or trelleborg (DK) or borgring (DK) or Ringwallanlage (DE). These were the type of fortresses built at from the end of the 9th century -many along the North Sea coast- against attacks of amongst others the Vikings. Also in the Netherlands -which most of it was then called Frisia- remnants can be found. But also in French- and Belgian-Flanders, regions Ostfriesland and Nordfriesland (Germany) and in Denmark.