Yeah it's slow and boring for a tower defense, especially if you actually played the old SC and WC3 TDs.
With that said, it's a very charming game that appeals to a lot of people, so it has that going for it.
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http://28.media.tumblr.com/tumblr_lcb35gGx0t1qailr4o1_500.gif
http://www.megavideo.com/?v=57N0YAEJ