This is a great game in concept, marred by some lack of thought in the execution. I would still recommend it, doubly so if you see it on sale. Expect to get frustrated and maybe not play through it all the way, but tower defense games are usually just time-wasters anyway, right?
This is a solid tower defense game, made unique by the collectible card aspect which allows you to collect your towers as cards, and to upgrade them by sacrificing other cards. Match two of the same card and you can "evolve" a card, and thus level it up (up to a max of level 3). Burn a pile of otherwise-useless artifact cards, or some lesser cards that maybe you have too many of, and you can increase the level of your cards, making your towers do more damage and adding range. The RPG aspects people speak of are pretty minimal, in my opinion. You, as a character, do gain levels, and this allows you to buy some abilities such as larger card carrying capacity, passive buffs, and more spells/towers to pick from during a battle. There are spells which, although pretty underpowered particularly in the second half of the game, add some extra bits of strategy when your towers are ALMOST good enough. The comic-book-esque cutscenes are kindof neat, tho the story isn't exactly going to be the driving force for playing this one.
The balance is pretty well thought out, with one very glaring flaw. If you are semi-lucky and get some good rare cards early on, or maybe if you stifle your spending of stars on your stats and instead spend the stars to buy card sets, you can pound through a pretty intense game. If, however, you don't get at least a lightning or dragon tower by about the third boss, the game becomes impossibly difficult because it's balanced for actually having these top-tier cards. If you don't have the rare towers, you may find yourself doing a fair amount of grinding, which really starts to suck the fun out of the game. There are a couple 250-wave levels which are a good means of getting cards and money and experience, but you can't access these until you beat the first half of the game, which really sucks when you just need to build up to beat the last few maps. They definitely could have put more effort into making the game adapt to the cards they've let you have, rather than assuming you're just going to grind until you get them.
A couple tips for this game: if you can hold off evolving a tower card to level 3, you'll get 5% more powerful towers if you evolve four level 1 cards into two level 2 cards, then evolve those into the level 3. Takes four of the level 1 cards, which can be tedious to get for the top-tier towers, but that extra punch helps for the second half of the game. Additionally, to help combat the wunky randomness of buying a set of cards for 3 stars and finding out you wasted your valuable stars, you can go to your %appdata% folder and back up the "Prime World Defenders" folder inside it, and restore it if you get dealt a bad hand.