In the last November I have had the opportunity to visit Edinburgh for a couple of days.
Short visit, but full of interesting things.
Edimburgh is a strange city, it is full of unexpected situations.

In example you can walk in the chaos of the main way and if you turn, for mistake or desire, in one of the Closes, suddenly you can find yourself in a space between the palaces where the voices and the noises of the main way are unable to reach you despite to the only 20 meters from the main way.
In one of these spaces outside from the time, the museum of the writers is found.
Settled during 1622 in memory of Robert Burns, Sir Walter Scott and Robert Louis Stevenson, all the guides will say you to visit it at least for the building itself.

But, if you want to understand as the Dr.Jekyll had to be felt during his work on the formula that would change him in Mr Hyde, try to walk around the evocative Closes in the middle of the night; stop yourself on th little particular of the buildings around you, try to breathe the atmosphere... but after, don't play with chemistry, of course!
But what are the Closes?
In the 1644 they decided to wall some poor quarters because of the plague.
The wiskey blurred the minds, it is known, and in this case also the memory, in fact they simply forgot about those quarters.
So over them, they started to construct a new level of the city, this level is now called Royal Mile.
Living in Turin (Italy) I'm quite used to hear story about underground galleries, but when they forgot that the Closes were an entrance of an underground world, they forgot also in that world persons were living.

Therefore during the jobs they had two Edimburgo: the new and bright one and the dark one, where the people lives under the official side of Edimburgh.
Then the plague thought to resolve this dichotomy of the city and in the decades a lot of voices about the ghosts of the old inhabitants conviced the people to stay away from the Closes.
Thanks to the fear of the plague and of the ghost, for 400 years the basement of the royal mile kept in a good conditions a lot of narrow lanes and houses of an entire quarter.
Obviously today the tourism does not have more fear of the plague so, in the Mary King' s close, the tourists can visit an house, considered by the guides, "one of the best existing examples of city architecture of the XVII century", all obviously flavored with a lot of ghost stories expecially of child ghosts for which people leaves flowers and toys in the close.
So if you are planning to go to Edimburgh, bring with you your ghost-buster suite!