The answer depends on your definition of a "castle," but the Czech Republic is home to Prague Castle, which holds the Guinness World Record for the largest ancient castle in the world and has roots dating back to the year 880 AD. However, many historians point to the Citadel of Aleppo in Syria (not in Europe) as the world's oldest. Within Europe specifically, the Alcázar of Segovia in Spain has foundations dating back to Roman times, though its current fortress form began in the 12th century. Windsor Castle in the United Kingdom is the oldest and largest occupied castle in the world, having been built by William the Conqueror in the 1070s. If looking for the earliest "stone" fortifications that resemble the medieval castle, France and Germany also boast structures like Château de Doué-la-Fontaine (circa 950 AD). While many countries claim the "oldest," the high density of well-preserved early medieval fortresses makes the region of Central and Western Europe the primary cradle of castle architecture, with the UK, France, and Spain holding the most prominent historic examples.