Samuel Johnson is explaining his idea of boredom and to explain it , he compares the state of being bored with the life in a city. First, he claims that there are people who can get entertained by themselves and , if they happen to get bored with this entertainment, they will bear it and even become reflective. These people have the mental capacity to do so. This is compared to a walled city that has got enough food for her inhabitants in case of a military siege. The walled city refers to the individual who can cope with his own entertainment and boredom, and the food with the mental ability to do so. The same applies to the comparison between people who depend on external factors to set boundaries to their stupid entertaitment or to keep them away from boredom ; and a city which needs foreing supplies to survive.
The main difference of simile and analogy is simile used comparison to make the description more emphatic and vivid ,( example : Sneaky as a snake, strong as a tank)
We can't find this kind of language on the paragraph above.
So, that paragraph above is more aptly described as an : analogy
