Restrictions:
Sadly it isn't using 84x48 pixel display... The sprites re low resolution, which is close to it, but when you move it is obvious that the resolution is way higher that the limitation.
Other:
I like the main mechanic and you made a great job on the visuals, but there are a few things that just didn't feel right. Mostly, the enemies. They aren't bad, but I would like a bit more of variation. Mostly they feel the same, since and your best bet is to just stay on the farther locations, since going closer to enemies has almost no advantages. This could be fixed by just having an enemy that uses area attacks. Maybe it's immobile and only attacks the furthest column. Having different enemies just add depth to the gameplay.
Overall, this is a great game, especially since it was a rather short time. It has some nice features and it is enjoyable as it is. Great job on this game, I had fun playing it. :)