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Is it the Fisher Price Great Adventures Castle game? That sounds identical to what you're describing. You collect knights (each of whom has a special "power" or ability) then take back the castle without actually fighting. The cover also shows a guy in green with a bushy brown mustache.
http://www.amazon.com/Fisher-Price-Great-Adventures-Castle-PC/dp/B000RE6DK0
Gameplay footage if it helps, but I'm completely sure this is the game you're talking about.
Yes that is. Thank you. The cover doesn't look like the guy you actually play but that happens kinda often with Video Games.

I don't really have an interest in playing this game but the name stumps me and it is getting annoying. Like having a song stuck in your head.
It was a Computer game I played in elementary school where you played a guy in green with a brown beard. You gathered knights by completing challenges and then non-violently stormed a castle to get the Big Bad out.