I just wanted to make a post to recommend this game to everyone who enjoys either tower defense games or defensive rpg games. This game is amazing. There is a huge focus on upgrading your character's abilities and towers. As you level up, you unlock new abilities. You don't get a choice in what abilities you get, they are decided based on your class - each class has different sets of towers and abilities.You can also upgrade individual items, such as swords or armor. There is also a minimal pet system, which is a familiar that hovers close to you and does various things depending on what type of pet you choose - some spit fireballs, others heal your turrets.
There is a vast variety of difficulty in the game - ranging from easy to insane. Upgrading your character allows you to better face tougher enemies, and you get better rewards from that and so on.
The map is divided into two different types of maps - normal maps and challenges. Maps consist of a specific amount of waves of enemies that come at you and try to destroy your crystals, and some maps end with a boss battle. Challenges are variations of the normal game mode, for example on one map you defend an ogre instead of crystals, on another map one of the team is marked as "chicken" and lots of enemies will seek him out and try to kill him - if you die too much, you lose that challenge.
The game is heavily focused on online 4 player coop. You CAN play it alone, but it isn't nearly as fun.
The game has a very arcadeish bright graphic design, some people will hate it but I think they pull it off excellently. Check out gameplay videos if you want to know exactly what it looks like.
The game has a demo available via steam.