According to the rules, the sum of all knights has to be equal or more than the amount of all cities and Metros on the board. If it is less, then all players with cities (and not only metros), who have the fewest activated knights, will loose a city.

Example:

Player 1: no City, no Knights

Player 2: one City with Metro, no knights

Player 3: one City, one Knight

Barbarian: 2

Knights: 1

Player 3 will loose a City.

Another example:

Player 1: 3 cities, 2 knights

Player 2: 1 City, 1 knight

Player 3: 1 City, 1 Knight

Barbarian: 5

Knights: 4

Player 2 and 3 will both loose a City.

*the number for the knights means the strength values of the knights....