Windsor Castle

Public transportation can get you there to join a tour. Make sure the queen is not in residence at Windsor. This closes it to the public.

You can prepurchase entrance tickets from the official website or just show up and buy a ticket at the gate. Take a train to Windsor & Eaton station and it's an easy walk from there.

Windsor Tickets:
http://tickets.royalcollection.org.uk/windsor-castle/windsor-castle/2017?_ga=1.102520833.1496443100.1488913656

You would take the train from Paddington Station to Windsor, you will change trains in Slough, it's a 35 minute ride. After departing the train, go to your left and walk up to High Street, across the street is the castle. The castle opens at 9:30am. The castle is open even if the Queen is there, she is there a lot of weekends. The State Apartments are closed at times for state dinners, Order of the Garter, but you can still still see Queen Mary's doll house. They are open next week. As you exit the castle there is a great pub across the street, The Horse and Groom. After you exit the castle and walk down to High Street if you turn to your left and walk down the street you will get to Windsor Great Park, there is another good pub right there. If you go into the park go towards your left up to the gates and you will have a great view of the Royal residence, in the park you will see The Long Walk, the private Road for the Royal family to use for coming to and leaving the castle. If you have time and want to, you could walk to Eton where Elton College ( actually a middle/high school) where royals and the aristocrats send their sons to. If you cross High Street ( to the side you come out to from the train station) as you face the castle, turn to your left, walk to the second street on your left and turn left, Ali a block then over the bridge and you will be in Elton. Just keep walking up th street and Elton College in on the right. Have a great trip.

Awesome advice. Thanks all esp Robin for the very detailed info!

What time would I need to leave London to get to the castle by 9:30 and can I buy advance tickets at a TI when in London?

Thanks MrsEB!