Standard Angry Birds rip-off.
Overall, it was pretty easy. I had to retry only 2 of the levels, and those were just to get crowns. 9/10 of the Rush levels (which, being last, I assumed would be challenging) could be finished with a single midlevel shot.
The powerups destroyed balance. Having constant fireballs really made it hard to take a non-metal level seriously. As a designer, limiting resources is one of your most powerful tools in shaping difficulty. Learn to use it; don't throw it away for the sake of giving upgrades.
Also, the racks on the prisoners felt pretty out of place for an otherwise childish game. But, maybe that's just me.