http://www.kongregate.com/games/m0rkeulv/castle-wars-2?acomplete=castle

The original one is quite good as well, and the hotseat feature made it a break favorite at work for a few weeks.

(the original was much more sandbox)

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There are a lot of deck building board games that started around 2008 with dominion that you didn't ask about. But a lot of these have really smooth mechanics that flow a lot better than a lot of computer games. It's actually kind of interesting looking at the rules that board games get, and how complex they can become with simpler rules. Lots of design skill to learned from here.

Edit: I do realize you said computer games, but this is what I know for card games. And you get three very different base rule sets with greedy kingdoms/Dominion/Ascension, even if Ascension boarders on MTG style monsters fighting type thing.

Risk-game with multitude of available cards(referral link):

http://WarLight.net/?Token=DNvNyJfdR3bzJkC

I think Age of Empires Online is using some kind of system like this. You have a hometown which you can boost with cards available in some form of store. Then you fight on the battleground, where the cards can give you some buff. I have not played the game so I'm not sure.