Schloss Marienburg

In the category of "extraordinary birthday presents" I give you Marienburg Castle:

The castle was built between 1858 and 1867 as a birthday present by King George V of Hanover (reigned 1851–1866) to his wife, Marie of Saxe-Altenburg. 

See, for more than a century the King of Hanover was also the King of Great Britain, so they had not invested very heavily in the court of Hanover, so when the houses were split in 1837 the Hanoverians found themselves lacking what wikipedia calls "a suitable summer seat." The architect was Conrad Wilhelm Hase, glossed as "one of Hanover's most famous architects."

So, this. But the castle did not remain in the hands of the Hanoverian royal family for long; in 1866 Hanover was annexed by Prussia, and they went into exile in England, where they resided until after World War II. They then returned home to assist in rebuilding Germany. Very little was done to update the castle during then interregnum, so it remained very much as it had been built.

The castle remained in the hands of the family until 2018, when its owner, Prince Ernst August, announced that he could not longer afford the upkeep and transferred it to the state of Lower Saxony.

Now it serves as many European castles do: weddings, conferences, movie set, general touristry.

The room you are most likely to see on the internet is the library. Notice the images of famous German authors and publishers; above, Sebastien Brandt and Johannes Gutenberg.

View up the tower, a photograph taken by (it seems) every visitor.

More. Will any billionaire of our time build a house with the lasting appeal of these nineteenth-century neogothic or neorenaissance palaces? I doubt it.