This was quite an enjoyable game. I got a bit stuck in getting to the second room, because the alien shot me the moment I came in, so I had to use the running through the collider-less wall trick the others noted in the comments. Then the third room was again a challenge until I realised that I could shoot into the next room from the previous one.
For me, the most fun room was the last one, because there was a bit of interesting problem-solving to be had, flying up out of the door and shooting as you go past. The other rooms couldn't be solved that way because of the horizontality of them. Maybe placing aliens above the doors in those rooms could have made them both more doable and more interesting (or barriers to protect you?). I also think being able to see where the aliens before they can shoot you (like in the fourth room) leads to potentially more engagement of trying to figure out solutions.
Overall, I liked this game!