This is a great bang-for-the-buck game that combines tower defense and first (or third) person shooter ideas into a fun little package. Worth catching while on sale. Fun to play with friends.
The under-advertised feature of being able to do local drop-in and drop out co-op with up to four players is a GREAT feature... and one I didn't even know was possible until someone mentioned it in game. You can utilize this with both offline AND online play. It's also a great way to power-level your own characters! Have a big bad level 80+ and you want to get a different class up in level and gear quickly? Plug in a Xbox 360 or Xbox One controller, hit start, and let them idle on a hard map you've mastered a defense build for.... Maybe even slap a rubber band on the shoot button, and let the game's auto-targeting hit anything somewhat in front of your lowbie. (The game seems to auto-target assist with a control pad by default... though not with a mouse and keyboard.)
The story isn't particularly great, though it is a nice touch. Once you figure out the "trick," many of the maps are pretty easy (though figuring out the best way to utilize your limited points for building defenses is one of my favorite aspects of the game.)
Unfortunately, there are points in the game where you will likely hit a gear curve, forcing you to repetatively grind a previously beaten map on increasingly harder difficulty settings to unlock better gear. Later levels and especially the expansion can be VERY challenging on "nightmare" difficulty. Or, you can look around at other player's shops and try to find some gear to buy. (Sadly, most players GROSSLY overprice the gear in their personal shops.)
The ability to unlock and select difficulty levels for maps is a definate plus. The 6 hero types (I'm not counting the mostly the same gender "alternatives") allow for different approaches to the maps, though true master builders will learn to combine the tower / defense types to really clobber harder levels and higher difficulty settings (a small hint: BUFF BEAM).
I spent a ton of time in this game and originally caught it a long time ago for like $5. It's been one the best game purchases I've made in terms of value alone. It's a pretty fun little game to boot. Go buy it already.