Skeleton Villages are the PvE mode in the game and skeletons are computer-generated characters. These are designed for you to practice your battle skills before engaging the PvP battles. 

There are total 6 level of Skeleton Villages. These are very tough skeletons and the following are their level equivalent to the cities. Please use them as a reference only because  the city layout of real players could be dramatically different and the amount of defending troops will be much bigger. 

Level 1 Skeleton > Level 3 City 
Level 2 Skeleton > Level 5 City 
Level 3 Skeleton > Level 8 City 
Level 4 Skeleton > Level 10 City 
Level 5 Skeleton > Level 11 City 
Level 6 Skeleton > Level 12 City