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CocosDenshion has now had some major upgrades including support for playing compressed IMA4 files, audio session management, compressed background...
Cocos2d wasn't used for GeoDefense although the developer is now using cocos2d. You are correct though, cocos2d's particle engine could be used...
I'm the developer of CocosDenshion. Please be aware that it has a slightly different license to cocos2d in that people/entities that are rich...