The Banqueting House (not really a palace or castle, I know, but let's not split hairs...)
This is an amazing place on Whitehall, absolutely steeped in royal (and republican) history. Designed by the great architect Inigo Jones in 1619, it's had many different uses and also has one of the most remarkable painted ceilings in the UK. It was used by Charles I, Cromwell, William and Mary and subsequent royals, and is now cared for by Historic Royal Palaces. It's also available for hire if you need a party venue...
The exterior in the 19th century...

And now... It really doesn't stand out on Whitehall, which is lined with grand buildings.

Once you go in, it's stunning.
Beneath the building is the undercroft, which has lots of historical information and is quite nice in its own right:


And what a history! Perhaps the building is most famous for being the backdrop to the execution of Charles I:

And it's also where William and Mary accepted the throne:

The best bit of the building? The Rubens ceiling in the main room, which used to host grand dinners and 'masques'







