Set in the world made famous by Majesty, in Ardania, you are the king. Some arrogant noble causes trouble by sending militia to attack you, so you slash back by building towers to defend your castle and sending various troops to counter-attack. Campaign takes you through Ardania to find the culprit of the attacks on your cities. Sounds cool enough, but unfortunately, you probably shouldn't get too excited.
Defenders of Ardania is a mix between Tower Defense (TD) and Reverse TD. You can protect yourself by building towers, but ultimately, you win by destroying the enemy castle(s) by sending out troops. Troops are quite strong: a single tower can hardly destroy a wave of enemies, so you either need to build more towers in clever spots or, which is usually easier against AI, destroy the enemy castle quickly.
There are about 5 different types of units and towers, none too different from other games of the genre. Some units can attack other units (rather weakly) and others can attack enemy towers. There's some strategy involved in placing the towers, as you are limited in the number of towers depending on the level. You may also direct your troops using a waypoint (yep, a single one) or place bounties (akin to Majesty but costs nothing) to focus fire on certain enemy. What is interesting is that your troops gain experience: using certain type of troops makes them stronger.
Later on, this feels indifferent though: it was always easiest just to either (following is a couple of killer strategies for campaign) zerg-rush the enemy with cheap or fast troops or use a reverse scorched earth tactic building a couple of waves of turret-destroying magicians followed by bomb carts that insta-kill castles.
This worked in almost every map: While AI can build towers to counter your troops, it's not up to these kinds of attacks.
Also, while there are three races, you can only play humans in the campaign, the rest restricted for AI and multiplayer (which I've yet to try: servers are empty though). Every race has its own towers, troops and spells, which is cool, but with mere single campaign, the game feels unfinished: there really should be a campaign for each race.
Story is quirky and and full of nerdy jokes, puns and references, so at least it does not take itself too seriously. You do have to endure some quite bad voice acting along the way (I'm looking at you, Kilia the Elf!). Playing it through lasts about 5-10 hours, probably less if you can come up with winning strategies faster. I guess it might be worth it for the amusing ending. :P
While not entirely bad, there are many better TDs out there: this one fails to fill its niche in the reverse TD. So if you just kind of like TD genre, stay away. If TD is your hot chocolate, by all means do get this one but do yourself a favour and do so when it's on sale or bundled.