Possibly the most impressive of all the English castles
The site itself dates back to the 11th Century, having been one of William the Conqueror's main fortifications following his invasion of Britain. Over the centuries, Windsor Castle has been added to, altered, and restored, making it one of the most expansive fortifications on the British mainland.
Basically, no visit to the UK is complete without a visit to this magnificent structure.
Pro-Tip: Because of its massive size and the multitude of exhibits, you should allow at least half a day for your visit. If you're a history buff, like I am, a full day would probably be better.