This is a surpisingly good little tower defense game with RPG mechanics. As a commander, you can recruit troops, equip them with weapons and armor and control their few skills when leveling up. It is pretty basic, yet very deep. The levels are single screen and grid based. You place your characters down and they automatically attack as enemies approach them. There are 6 classes, a basic melee, long range, healer, ice mage, slow defensive breaker and all powereful dragon with short and long range attacks. It costs PSI to place your characters, and more PSI to retract them (to move them).
How do you get PSI? Well you start with it, but then as you kill enemies, you gain more PSI and you have a maximum of course. You are defending a summoner against the waves of enemies and the summoner (your player character) can take a beating and has plenty of HP, so the game is forgiving, but rewarding if you make it through the level without being damaged.
You can 'boost' your characters at the cost of PSI. There are many levels of boos that enable skills that you've unlocked through leveling up your characters. When you level up your characters, you select their perk. Which has a simple perk tree of skills and passive abilities. It is a nice little system, but I find myself making the same perk choices instead of diversifying.
Oh and death is not perminant for your characters. If someone dies, just use PSI to bring them back. If your player character dies, you lose the level and need to restart. These levels are quick, especially at 4x speed, so nothing is really lost.
While the game boasts a rich story.... I found myself hitting the skip button. I just don't play games for the story. You read the story as you unlock new levels and progress through a Mario World style map with all of the levels plotted as map points.
The game is very easy to pick up and play, but not just that, it encourages replaying the levels with 3 different difficulties (and a 'casual' difficulty as a 4th). If there's an insurmountable level.... replay old levels *cough* grind. But replaying old levels never seems like a chore, its more of a joy. Even if you keep replaying the same level on the same difficulty, every playthrough is different, since your characters level up and you can change your tactics.
The game is a lot of fun, and while its deep, its never overwhelmingly deep. There's not an overwhelming micro management either, even if you can micro manage everything.
I highly recommend it. Its a fun little game, relaxing and never gets all that intense. I found myself playing it on 4x speed and that seems like it should be the average speed. Is it worth the $15? Its a game you can sink hours and hours into, so you'll get a lot of relaxing playtime for your dollar.