The Brittenburg (Classical Latin: Batavorum Lugdunum) is a Roman ruin west of Leiden, presumedly of the even older Celtic Lugdunum fortress, that was visible on the beach between Katwijk aan Zee and Noordwijk aan Zee after storms in the years of 1520, 1552 and 1562. It was originally a large complex located at the mouth of the Oude Rijn (old river Rhine), which today is believed to be about a kilometer westwards (offshore in the North Sea) of the current location of the European Space Research and Technology Centre.