The planetary missions currently look like placeholders and I doubt players read the text after the first few. If the missions had some game elements that had some risk and reward (double the reward vs. lose 5 crew) that would make them more interesting
In terms of level design, this is one of the best physics puzzle games on the site. In order to appeal to a wider audience, the designer may want to give in and use cute characters instead of abstract shapes.
The best defense is a large population. Each dropship kills 200 of the population on a planet. When that gets below 0 you lose it. If you can keep the population above say, 1,400 then it will take the enemy 7 dropships to get through.
This has an aspect that other TD games have, where I lose motivation to finish all the way if I complete the final level on 'easy'. I'd like there to be a true final objective that requires me to finish every level on the hardest setting. Getting yet another badge doesn't seem adequate reward for that.
This owes so much to Kingdom Rush that you may have to call it a "tribute" or "homage" or something. My one complaint is that the theme of child abduction seems unnecessary, especially since the player is the abductor. You could have the "gems" be candy or something like that, while maintaining everything else.
It's grind without any actual strategy. The graphics are nice, but the player never has any real choice about what to do. You max out upgrades and then move on to the next creature. And you have to fight the same guys every time
Like others said, it's too easy to build towers and let them do all the fighting until you have enough reserves to crush the enemy in one swoop.
So, there's a balancing problem and not enough strategy. There neds to be more of a struggle to build your defenses - perhaps only allowing one tower initially, then 2 after the 4th level, etc. And the XP was way out of balance. I maxed out all the research by level 6. It was too easy to set things up, walk away for 10 minutes, then tweak a few things and walk away again, then return and find out I won.