Nice game!

The lack of view felt like a constraint, but I suppose that this was intended. The game is "too hard" at first, because it's impossible to protect all your trees, but after some time it's "too easy" when you just protect 3 trees in a corner, and the machines are too slow to be an issue. I went to 150 kills before I decided to give up and see the death screen :p

A great little game for your first ever creation! I hope this is the start of many more :)
To improve this game, I would zoom the camera out so we can see the whole oasis at once, look into adding some variety to the game play, like different enemy and weapon types, and look into adding music and sound effects. Even the simplest effects help tremendously to make your game feel fun to play. 

Keep it up!

It was fun! With some upgrades of the weapon it would be even better. 

I like the idea to defend the Oasis. The controls are nice responsive. I like it. Good Job!