In its day, there were two European castles that were contenders. One was the Citadel in Constantinople (112 acres) and the other was Carrickfergus in Llewellyn Wales, also 112 acres. Both now exist as ruins. Both were Roman in origin but I would not wish to stake my reputation on the source: Bailey's Points of Interest (2010). I have been to Elsinore in Denmark. Magnificent at least!