I tried to find the answer in unity3d site but without success. If the answer to my question is yes, does anyone know where I can find an example?
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of course you can, nothing is impossible. some things are just darn hard. And you are not going to find perfect examples of HOW to do specific games unfortunately.
My reasoning is this: if you could develop a 2D tower defense game, you wouldn't have taken time out from developing a 2D tower defense game in order to ask us whether you could develop the 2D tower defense game you were already developing.
Hope this helps.