Art: I like it, only thing I'd change is make some enemies a bit brighter, they can be a bit hard to see on the dark background.
Music: Fits the game. And it changes at the boss fight, which is nice too.
Writing: done by a sound effects person? (Saying this because I read the credits screen.) I think the story is told without words, so I would probably have used a different word (maybe "script", although in programming, that's a different thing... so maybe "story" or "script" depending on how detailed the writing was).
Suggestion: end with the menu, rather than a black screen (I know I can get the menu by pressing escape, but I'd have preferred to get it automatically, or the game to just keep showing the "thanks for playing" message so I didn't wait several minutes to see if anything else would show up). And since the credits screen can fit on the screen all at once, maybe when the player chooses it from the menu you can show it without the animation and just with a button to go back to the menu.