I found this game quite a bit more enjoyable than I had initially expected it to be, considering the relative simplicity. Combat has some added strategy given the delay on player attacks, a lack of healing/regeneration provides a challenging aspect when it comes to racking up a high score, and the game overall feels very consistent (I can tell when my attacks land, I can tell when my attacks get interrupted, I can tell when the enemy attacks land, I can tell when the enemy attacks get interrupted, etc). The foundation of the game feels rock solid, which is a LOT more than can be said about many different games that cost actual money to play.
When it comes to improvement, I found myself longing for "things to be added" much more so than "things I'd like to see changed" which is a good thing in my opinion. The only real exception to this is the "combat idle mode" that is already implemented. In my play-through(s), I didn't notice this having any effect on my character. I had anticipated this to allow my character to heal/regenerate health, however slowly, or perhaps build up strength for my next attack; something along these lines. I think either would be good, as the game has a distinct lack of survivability, but if that is an intended aspect of the game then perhaps being able to use this to empower the next attack unleashed by the player would create some more dynamic gameplay.
Aside from that, what I noticed missing from the game above all else was sound: background music, sound effects, things like that. While it is nice to be able to play and listen to my own music simultaneously, my review wouldn't be complete without mention of this.
The only other issue that I had with this game was the wall bug: enemies have the possibility of spawning outside the boundaries of the game that the player can maneuver in. This isn't too bad when it's just one, maybe two enemies, but if I'm backed into the corner (unlikely with the boundaries being as big as they are, but possible) then I encounter the issue where 3 enemies spawn somewhere that they cannot approach me, and the wall is too thick so I can't kill them to make them re-spawn in-bounds, which essentially soft-locks the game since no more than 3 enemies seem to spawn at any given time. An unlikely issue, but one worth mentioning all the same.
In summary: good game. The foundation is incredibly solid, there are no glaring issues or design choices that hold this game back, and as it is I think that this game is on the border of being complete/a finished product. Fixing/enabling the dynamic gameplay mechanic that's already been implemented, adding some sort of sound, and removing the possibility of soft-locks all feel like rather minor adjustments, and yet they are the most that I can criticize (which is a very good sign to me!)