The game was pretty enjoyable. The retro art was good, the sound and music were great as well (Although i don't know which assets are yours and which are from online, so you should credit the stuff that you used. Also, try to avoid using copyrighted assets, like the spider sound).
The gameplay was quite simple but nonetheless fun. The style reminded me a lot of Mario. The difficulty balance was good, but you really should add the "load" button to the controls (I only found out about it because i watched the stream). I loved the cutscene at the beginning with the voice over :D
I only found some minor bugs while playing, you can spam the "load" button to get stuck in the air, and sometimes if you duck down and go left or right you can walk and even sometimes go through the floor. On the cutscene, you could still click the buttons of the main menu, and i also got stuck on it for a while since there was no indication that pressing space would skip it.
All in all, a simple entertaining game. I feel maybe the biggest downside with the game is that it's a bit too short. Good luck in the jam!
Important Note:
I think it's really cool (but needlessly difficult) that you decided to try making this game without an engine. I have personally never made a game with SFML before, but there has to be a way to package up your assets and code so that they are not so easily accessible to anyone (someone could use your project as their own), plus it'll make the game folder a lot less crowded. Don't do that right now though, you can't change the jam submission before the results are out.