A whole lot of content for a game that's free. It's considerably challenging and satisfying to succeed at. Each stage has well designed puzzles and status conditions which make the gameplay fresh and different from other stages. I rly have no complaints. 

Expect to die at the start of each stage as you learn tactics for success, but this is built in as part of the game mechanics so there's no reason to feel disheartened. Keep pushing forward, use your brain, and you will have a blast.