I have recollection of looking at a memorial plaque about it,
Oh I did, it is this one here
, so I must have ventured further onto the island than I remembered.
The German site with one million visitors must be referring to something that got a boost from becoming a World Heritage Site rather than an equivalent communication heritage site?
From looking at our connection of sites with over a million visitors,
the best guess I could make is Zollverein
(incidentally the link on that one is a deadend now, but the official site
has a figure of over two million visits), as the others don't seem comparable; Museuminsel, Aachen and Cologne Cathedrals.