Thanks for playing!
I struggled with how much guidance to provide. Initially I had a bunch more levels with each step spelled out explicitly, but I felt like it was too much hand-holding, like a series of fetch quests which turned into a chore, inducing players to just do the thing as quickly as possible to get to the next level. So I took them out and made it more "do whatever you want."
Perhaps a happy compromise would have been to provide occasional hints or achievements.
Fend off an alien, protect a city from asteroids, etc.
Also, a lot of intended content was cut due to time constraints, fear of introducing new mechanics at the last minute, and developer sanity.