We have recently visited Windsor Castle which is in the town centre of Windsor. Its really well signed and there are lots of car parks near to the castle but they are pretty expensive though so take lots of change.
Windsor castle is the largest inhabited castle in the world and is one of the Queens official houses. It was originally built for William the Conqueror.
We arrived just as they were about to do the changing of the guard which was great to watch with marching and music etc. They do this when the Queen is in residence which luckily for us she was, although we didn't actually see her. James was far more interested in following the Queens cat Mimi around rather than watching the ceremony.
We headed first to St Georges Chapel where many of the most famous monarchs are buried such as Victoria and Albert, Charles I and Henry VIII and Jane Seymour and many others. It was interesting inside and a beautiful chapel. There were guides in the area who were really helpful and great with the kids, pointing out different things to them and explaining them.
We then headed to see Queens Mary's Dolls House which for a dolls house is massive and comes complete with vehicles, petrol pumps, a room for the Crown Jewels and a throne room among many other things.
We then went on to visit the State Apartments where we saw famous paintings, Henry VIII's armour, various outfits worn by the Queen, rooms full of suits of armour and coats of arms, a whole room dedicated to Waterloo and Wellington and another to Nelson.
The castle has many features you would associate with a classic castle such as cannons, battlements, a portcullis, arrow slits etc and the boys really enjoyed pretending that they had to defend the castle.
If you are making a day trip to Windsor we would also recommend Windsor Great park and Viriginia Water which are nearby.