Damn!... I wanted to lose at 666, but the game continued increasing my score for 2 more seconds during the death animation. So close...
Nevertheless, it was quite good! I felt my heart rate increasing as I was getting closer to my target score. The skulls and their movement trails also look good; the aesthetic would've been even better if the pixel size of the background matched the pixel size of the skulls (rule #1 of pixel art games).
Also, I think the enemy skulls stopped spawning at some point but it would've been more challenging if they had kept spawning forever (or there was a difficulty setting for that) because my survival depended more on my patience than on my skill. Or maybe one/some of the enemies can shoot in order to spice things up.
And I'm not sure how well this would play out, but maybe the enemies telegraphing their next movement would make the game more fair. Or, perhaps, their opacity/hue can be based on how far in the past they are so that their train-like movement can be more obvious to the player. I think the former would be more useful, but I'm not sure what a good way to implement it would be...