Looks nice, fun to play. Very cool to see your space marines laying down the fire on a wave of martians. Poor martians. It is their water, after all.
But the problem isn't so much that it's CHALLENGING, as that it is UNFORGIVING. For example, if you die on the last stage of a day, you will have to start that day again from the beginning. After dying on the last stage of day 3 and learning that I would have to play the entire day again I rage-quit. Deciding to give the game another chance, I loaded it up again later, only to find that my progress is lost - I have to start from day 1. This is despite the fact that it saves your lineup of soldiers. How can you try out different strategies if you have to start from the beginning every time?
Another example of unforgiving - it costs money not just to recruit soldiers, but also to deploy and un-deploy them. Not exactly encouraging strategic exploration, is it?
All in all, when making another game or updating this one you need to remember that people will play this to have fun - not to bang their head against the wall. Adding different difficulty levels would allow you to make the game you want, as well as allow people to play it the way they want, which is a win-win situation.