An unexpectedly enjoyable experience, given how short it is, but I don't think it needs to be longer to get its point across; if you think of it as a subversion of Mario, it even makes sense that it ends immediately after you reach the "first" castle. After all, you'd expect it to continue after that because an actual Mario game would, so the abrupt ending is fitting unexpected!
That's not to say that I wouldn't enjoy playing an extended version, of course; the muted Gameboy-esque palette and classic sound effects are strangely soothing and knocking the planks off the door in the very beginning was oddly yet immensely satisfying. It also feels like there's something going on in the backstory, what with the house being boarded up from the outside and the "There he goes again" attitude of the princess and the not-Goomba if you knock yourself, but maybe that's just me?
Anyway, I had fun and it's clearly well-crafted despite is brevity. Good job!