Anomaly Defenders is a nice little game. I've played through 7 of the 24 levels, all on Medium difficulty (there is also Easy and Hard). Can't really compare it to other full-fledged TD games since I am not much of a fan of the genre, but all I can say is that it is a good TD representation of the Anomaly series.
The towers and the human units would be familiar if you've played the previous Anomaly games. Defenders builds upon that, and you have abilities as well (similar to the previous games, again). You can upgrade units, and sell them on the fly. There is a skill tree too that uses points you accumulate after every successful mission, where you have some amount of freedom to chart your way forward.
One nice feature is that you can pause the game to strategize, and also build towers, apply abilities and upgrade all while the game is paused...too many TD games are just so action-packed you don't have time to think at all.. But that's not the case here.
It's kind of obvious Anomaly Defenders had less work put into it compared to Anomaly 2 judging by the graphics and the lack of story, but the good thing is the gameplay doesn't seem to have been truncated and the price is at least reflective of the smaller effort put into the game. 24 levels seem quite a bit, and you can replay earlier levels on harder difficulties when you've unlocked better upgrades in the skill tree.
Overall, if you were a fan of the Anomaly games prior to this, I'd recommend Anomaly Defenders. Decent amount of content, strong gameplay (even if it starts off too easily) and a reasonable price-tag.