Tried it yesterday. I did not know the original game.
Quite fun and pretty hard indeed. After several tries I could only reach level 5.
I usually lost most lives on level 3 (due to the spawner).
Not sure if you are going to make some revisions of the game but would it be possible to have the first spawner you encounter in the game spawn enemies at a slower rate? With the spawning rate increasing in the next levels?
If that is already the case I guess the game becomes very difficult after a while.
Or perhaps limiting the number of spawned enemies for the first level with a spawner?
Two times I had the spawner behind a wall, near the vertical center and therefore, as I could not attack it quickly, it had time to spawn a lot of enemies and when I tried to attack it, I could barely reach a safe distance to shoot at the enemies coming at me. Perhaps some diagonal attacks would have helped but I had not really enough time to think about it.
Oh and this game is vicious. Several times I thought I had finished a level because I had just killed the last enemy but was finally killed by the last bullet it shot
Anyway good job overall. Highly recommended to anyone and especially for people who want to train their close quarters fighting ability in games like the Chaos Engine