Cool game, I really enjoyed it!
The game plays quite well (smooth gameplay, tight controls), the goal is simple to understand but the game rapidly gets more challenging (when the enemies moves faster). However I think that the coolest idea of your game (the tombstone) may be "under-used" for now (that's my personal feeling).
For example, have to tried to have the player die when they touch a tombstone? That could add more "hazards" to the game with each shot, and so force the player to think twice before shooting an enemy. For now, I feel that I can shoot them all down as they appear, because the only "negative impact" it have on me is that it will block my way, knowing that I can easily go around them. But If I had to also avoid touching the tombstones, I think I would be more careful before shooting an enemy.
Looking forward the next beta!