Additional feedback after a bit more playtime. The jump in challenge from one difficulty mode to the next feels like a huge spike (and it specifically says that from the lowest to second difficulty is literal doubling of enemy power). This is also reflected in the endless high score mode, which also feels like it ramps the challenge up a bit faster than I'd like. Toning the gaps down in difficulty, at least on the lower levels, would be nice to see in future.
VERY cool game though, even if it feels like it could do with some less extreme jumps on the lower end of the difficulty scale.