Thanks a lot! I did want to make the difficulty ramp a bit like a sawblade (_/|_/|_/ kinda), having the higher numbered levels being harder than the easier ones. I wanted to ensure that the player would master the mechanics, and specially got some practice with rewind, so that whenever new turrets or different level designs appeared, the player could handle themselved easily. It's more of a set of levels and mechanics than a continuous flow of gameplay from 1 to 15, that's why the levels are both separated in 3 different menus and why completing level 1-5 doesn't take you to 2-1.
I'm glad you had fun with it, thanks for playing!