I am a big tower defense fan and have played almost all of the games on my computer. Now I am looking for the good and challenging TD games for Nexus 7 Android tablet and that could match the graphics and realistic.
Towers N' Trolls by Ember Entertainment is a free tower defense game for Android, like Warcraft: Orcs & Humans. This is the best addictive game I have played on my Nexus 7. It comes with lots of challenging levels and some lovely aesthetics.
Compared to other TD, it has wonderful graphics and awesome sound and easy to learn but slow speed. I play it smoothly on the tablet. However, if you want to compete the worlds, you need to purchase new levels and unlock the maps.