this game is an online game which you can play for free, it is really very addictive with 5 levels of games. you can see two versions below.

Play Desktop Tower Defense 1.5

Play Desktop Tower Defense

The objective is very simple any one can play this. Just give a try (Don’t get addicted ;) )

Actually it is an old game, first edition released on March 2007, almost 2 Years back. Still it is good with newer versions. DTD is available in an English, Spanish, German, French, or Italian interface.

here is a Wikipedia article