Stronghold: Crusader

Stronghold Crusader is the sequel of Firefly Studio's Stronghold.
It is a 2D RTS and city building game and Crusader offers several new things.
For example Skirmish mode, mercenary units and several campaigns.

This game requires some serious castle managing skills and also military skills.
First you have to build a strong castle and a good economy in order to get more people in your castle. Later, with enough resources, you can start recruiting your soldiers.