This is shaping up to be a fantastic conversion of the arcade classic. I played the latest build at the end of my Twitch stream (CharlieFar) last night and played about 20 waves. It looks to me like the difficulty level isn't progressing the way it should. By level 4 (I think), we should start to see up to 4 descending enemies at a time (excluding the 2 red and yellow ship formations). It remained at 2 attackers only all the way through.
There was a couple of other small issues: the background drone tempo was wrong at times and occasionally enemies would disappear without showing their explosion.
As I said, this looks like it will end up being the definitive port. I'm the current arcade world record holder on this game, so thank you JOTD for taking on this project.