It is too difficult even in the first level. I cannot hit the second alien before he attacks me

This is some pretty promising work! The controls were pretty tight, although the jump was a bit floaty. The explosion particle effect was a good damage indication, although there were times when it wasn't clear if I was supposed to shoot something or not, simply because it took a lot of hits and didn't move (i.e. the tower in level 2 and the boss alien in level 3). The enemies could also be pretty sporadic and hard to dodge. That being said, the gameplay was generally fun and I like the variety in the levels. This game is really close to a polished work; it really just needs some graphical updates and a few gameplay tweaks. I also really like the fact that you went for an overarching story, as most Ludum Dare games forgo that entirely. The intro and outro were pretty cute and the menu looked really great. The story really just ties the whole thing together nicely. Also the music was really good too. Overall, I enjoyed my time with this entry. Good job!

I really like the screenshots, but when I try to download it my computer seems to think that it is some sort of textfile. Am I making something wrong?

@christian-koch You need to extract the files using winrar or 7zip.

Difficult is too high, I can't beat even 1st level, but I like simple pixel graphics :)

A well put together game, I especially enjoyed the visuals and music. Great work.

"You saved the world!" That's why I love hardcore 2D ACT platformer. Very challenging and fun! (Charging aliens are the worst! Hard to deal with)

Nice game. I didn't make through lvl1, to difficult for me.
I loved movement, very tight controls, also sound and music was great.
I didn't like rushing aliens, and the fact that I died to much! :)

Very challenging game! I included it in my Ludum Dare 38 compilation video series, if you’d like to take a look :)
https://www.youtube.com/watch?v=jKyqjTvBURc