The game is playable without needing to spend any money. Every stage is a puzzle and it just has a higher difficulty than if you spent money. This could be more fun for some people than if they had better units and stomped stages. People routinely do hard content with lower rarity teams. I play pretty casually and spend at most 5$ a month usually. I don't think I've been specifically lucky and I have a large majority of the units in the game. The story is told through a visual novel style and is generally pretty good considering its platform. I definitely suggest this game over most if you wanna play something on your phone. Most are far more predatory.