Brittenburg was a Roman ruin site west of Leiden between Katwijk aan Zee and Noordwijk aan Zee, presumably identical to the even older Celtic Lugdunum fortress.^[1] The site is first mentioned in 1401, was uncovered more completely by storm erosion in 1520, 1552 and 1562, and has subsequently been entirely eroded away. When built, it was located at the mouth of the Oude Rijn (old river Rhine), which has since moved. The site was about a kilometre west of the European Space Research and Technology Centre, now offshore in the North Sea).

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The word dunum, traceable in Gaelic place names in the present day (Dundalk, Dunrobin Castle) and meaning "fortress" or "castle", is a typically Celtic element in European place-names. The site, known as "Brittenburg", was still visible in the dunes in the fourteenth century, but the gradual advance of the sea made the ruins lie on the beach in the sixteenth and seventeenth century. Today, they would be somewhere in the North Sea, inaccessible to archaeological research.^[2] All that remains is a small set of finds, collected in the sixteenth and seventeenth centuries, and a famous map by Ortelius. A copy of an old Roman map Tabula Peutingeriana shows Brittenburg as Lugduno; on the coast and with two towers. Eastwards from that point, two roads run towards Noviomagi (Nijmegen). Along the northern route, the following towns can be seen: Pretorium Agrippine (Valkenburg (South Holland)), Matilone (Leiden), Albaniana (Alphen aan den Rijn), Nigropullo (Zwammerdam), and Lauri (Woerden). All of these locations are situated on the Oude Rijn. The southern route begins with the town Forum Hadriani (Voorburg), shown directly south of Matilone. These towns were connected by the Fossa Corbulonis or Corbulo-canal.^[3]

The first mention of the Brittenburg in a Dutch text is in a poem of Willem van Hildegaersberch in 1401, who called it Borch te Bretten. In 1490 there is also a mention of the visibility of the "burg te Britten". It was uncovered in 1520 when a storm exposed the whole complex and Roman artifacts (mainly stones and coins), were found. Some coins were dated, with the latest date being 270. The oldest picture of the Brittenburg is a woodcut (identified by Leiden professor Jan Hendrik Holwerda, curator of the Rijksmuseum van Oudheden) by Abraham Ortelius in 1562 for Lodovico Guicciardini's first edition of The Low Countreys, printed in 1589 by Christophe Plantin in Antwerp. This woodcut was replaced in later editions with an engraving. The oldest picture was used later by Zacharias Heyns (1598, 1599) and Hermann Moll (1734, 1736). It concerns a land surveyor's draft (trigonometry), in which the distance from the ruins (by that time in the North Sea and only visible at low tide) westward to the church of Katwijk is mentioned, namely 1,200 'schreden' (= 1,080 meters). Brittenburg was part of the Roman border defense (limes), as the guard post (castellum) called Lugduno, the westernmost position situated along the Old Rhine, which formed the northern frontier of the Roman province Germania Inferior. Given the square shape of the inner structure, the Brittenburg was probably a lighthouse after the model of the lighthouse of Ostia Antica with a height of about 60 meters and a basis of 72 x 72 meters. Some historians also see a granary in the plan, but for this a drier, more inland location would have made more sense.