This is my favourite game in years.
It's a tower defense game where each of what would normally be a tower is actually a character that levels up, like in an RPG, complete with a customizable skill tree, equipment, etc. These levels are earned from level to level, so you're constantly improving your party.

This superficially simple gameplay concept is backed up on all sides by a great story, deep strategy elements and amazing replayability.

Story
Azra (or the name you choose) is the royal librarian that ends up in The Pit, a wretched place full of zombie-like creatures called Revenant. Using mysterious special powers (that excellently explain the tower defense gameplay) and the help of a band of ragtag adventures you'll pick up it is your job to guide her out of the pit. There is more at stake, and the plot has plenty of developments to keep you interested, but what surprised me the most were the story's pacing and its humour.
No story segment lasts more than a minute or two, excluding the ending but not the opening, yet it tells a cohesive and engaging story. You'll never feel like the levels are just there to pad out the story or vice versa.

Strategy & Replayability
Within levels you get to place your defenders along the paths your enemies will take to reach Azra. Doing so spends mana, so unless Azra has a lot of Mana (maximum mana increases by increasing her level) you'll need to consider which allies to summon. You can 'boost' these allies to unlock more attacks and increase their core stats, up to 5 times, this is equivalent to the traditional tower defense game's tower upgrade. Combine that with being able to place allies whenever you want, advanced targetting settings and certain allies working very well together while others negate some aspects of other powers (frozen vs. burning) and you have a deeply rewarding tactical system.
Is this required to enjoy or finish the game? No.

The ability to play and replay any level you've cleared at any of 4 difficulties means that you set your own pace and your own challenge. On the lowest difficulty you can basically breeze through the game and just enjoy the story, never having to put a lot of thought into tactics or grinding, and the hardest difficulty will have you either rethinking your tactics or grinding a few more levels.
Clearing the game once, with any of its 5 endings (only 2 distinct ones really, one just has 4 different versions) will unlock new game+ mode parallel to the regular game mode. This features new powers for all enemies, several sidequests telling the story of one of the characters relevant to the plot, the ability to understand the 'ancient language' spoken in some of the dialogue and a 'journal' that offers lovely (and often hilarious) glances into the protagonist's mind as her journey progresses.
It will challenge you and reward you, and this is completely free extra content that was released months after the game was originally launched that is automatically included. I've put dozens of hours into this game and still revisit it regularly to see if I can't beat a few of the levels I still have problems with on a higher difficulty or to work on my Hero Mode run.

Summary:
+great plot
+great tower defense gameplay
+great RPG-style customization
+replay levels
+multiple difficulties
+multiple endings
+best way to do new game+
-graphics are serviceable, but nothing to write home about
-no voice acting (which to me is better than bad voice acting...)
-Defender's Quest 2 isn't out yet, give it to meeeeee!