Hi, I really like the game so far, and am quite eager for future updates! As a token of my gratitude, I leave you with a small bug report, and a couple of suggestions for future updates.
When an enemy begins to flash, if you time an attack right, they will either pass straight through you without dealing damage, or you can hit them regardless of your position or facing.
In the future, I would like if all enemies could be toppled, as I feel it would simply add more variety, but the animations that you made so far are excellent!
Also, I think it would be nice if the color of each level were different. Like if Tunnels I started with the blue you already have, and then Tunnels II had perhaps a darker blue, or even purple. It could make each level feel a bit more distinct from one another, and would add a sense of progression.
Once again, love the game so far, and I hope you have fun making it, and produce many updates in the future!