Okay, you have a fun game on your hands! I am about half way through the 3rd campaign.
The first two campaigns (stories?) had a good feel to the progression. On average, I needed to earn about 2 stars per level to get the tools that I needed. The story itself is engaging and interesting in how it relates to the magical crystals! The most important unlock for every story is the building destroyer projectiles. These weapons help you kill stuff faster and earn more money, making your progression possible.
I think you should reconsider the difficulty of the third campaign. The issue i noticed was how terrible the mailed fist is at destroying walls/rooms. I find that strange because that is the job that this item is supposed to do! I was stuck for some time in the third story until I fluked out and got enough stars to get a better projectile. I feel like the story is losing it's focus a bit in the third campaign, but it may still come back at the end. I should say, what i found most difficult was the time limits on the castle assault missions. With even 1-2 more minutes, it would be a more enjoyable experience.
That's all for now. Thanks.