Windsor Castle is just 8 miles from Heathrow Airport.

Next time you go to Heathrow Airport, Why not take a small side-trip to WIndsor Castle ?

• Its just 8 or 9 miles (12 to 14km) away (depending on which Terminal of heathrow).
• If you have a car (or taxi), it will take about 15 to 20 minutes. (Uber Taxi is around £15 to £20)
• If you take a bus (from Heathrow terminal 5) it takes about 45 mins to an hour (depending on time of day).

  bus number 8, (Terminal 5). Bus runs every 30 mins (office hours) and then every hour after that.
  Timetable can be found here:
  http://www.londonbusroutes.net/times/008Heathrow.htm#1_C
  (get off at Windsor Parish Church. The castle is just 1 minute walk up the road from the bus stop.. Follow the trail of toursits).

• It's not worth getting the train as there is no 'direct route'
  (it would take at least an hour or more, needing 2 or 3 different trains).

Windsor itself is a lovely old town.
But, if you are 'against the clock' then you will need at least 1 to 2 hours to see what the Castle has to offer.
(of course, you spend up to 5 or 6 hours in the castle if you want to take in every detail).

Entrance prices:
Adult £21.20
Over 60 / Student (with valid ID) £19.30
Under 17 / Disabled £12.30
Under 5 Free
Family £54.70 (2 adults and 3 under 17s)

If the 'State Apartments' are closed, then this is the price:
Adult £11.70
Over 60 / Student (with valid ID) £10.60
Under 17/ Disabled £7.20
Under 5 Free
Family £30.60 (2 adults and 3 under 17s)

You can buy tickets at the door (if you dont mind queuing), or you can book in advance online here:
https://tickets.royalcollection.org.uk/windsor-castle/windsor-castle/2018

• Dont forget to look inside St George's Chapel within the Castle grounds.
  Some famous Monarchs are burried within the Chapel (including Henry 8th, Jane Seymour, Charles I (famously beheaded by Oliver Cromwell), and both of the current Queen's parents: George 6th and Elizabeth.
  Also: Prince Harry and Meghan are due to marry there in May 2018.

• Make sure to see all of the State Rooms and the Queen Mary's Dolls' House.

NB: I am NOT including photos of the St George's Chapel, State Rooms and Queen Mary's Dolls' House because I don't want to spoil it for you.
I like the idea of 'trusting' that it will be impressive, and going there to see it with your own eyes, without spoilers.