This is the 694th most frequent Finnish word.
kaupungin
"Of the city" (genitive form of "kaupunki").
Here, 'kaupungin' is again in the genitive case, describing whose center it is, i.e., the center of the city.
Kaupungin keskusta oli täynnä ihmisiä.
The city's center was full of people.
In this example, 'kaupungin' in the genitive is used in a broader context, suggesting the city being referred to in the map finding activity.
Hän löysi kaupungin kartasta nopeasti.
He quickly found the city on the map.
In this sentence, 'kaupungin' provides the noun it modifies a territorial specific, delineating the central area of the city.